Question: What era was the well known buliding on the border of United States and Canada declared a heritage building by both?

full_context: Document 1: Line house: A line house is a building deliberately located so that an international boundary passes through it. The term is perhaps more frequently used in reference to the boundary between New England and eastern Canada than to other places. The most well known such building on the boundary between the United States and Canada is the Haskell Free Library and Opera House in Stanstead, Quebec and Derby Line, Vermont. The border is marked on the floor in a reading room and an auditorium. A number of single-family residences and some industrial buildings straddle the boundary in those two towns.
Document 2: Adams Memorial Building: The Adams Memorial Building, now also known as the Derry Opera House, is a historic municipal building at 29 West Broadway near the center of Derry, New Hampshire. Built in 1904, it is a remarkably sophisticated Colonial Revival structure for what was at the time a small community. The building originally housed a variety of municipal offices and the local library. Local events are occasionally held in the theater ("opera house") of the building, located on the upper level. It was listed on the National Register of Historic Places in 1982. The theater is now operated by a local nonprofit arts organization.
Document 3: Grand Opera House (Ashland, Wisconsin): The Grand Opera House is a historic opera house in Ashland, Wisconsin. The building was one of the first theatres in the region, and was originally a vaudeville/opera house/live performance venue.
Document 4: Canada–United States border: The Canada–United States border (French: "Frontière entre le Canada et les États-Unis"), officially known as the International Boundary, is the longest international border in the world between two countries. It is shared between Canada and the United States, the second- and fourth-largest countries by area, respectively. The terrestrial boundary (including portions of maritime boundaries in the Great Lakes, and on the Atlantic, Pacific, and Arctic coasts) is 8,891 km long, of which 2,475 km is Canada's border with Alaska. Eight Canadian provinces and territories (Yukon, British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, and New Brunswick), and thirteen U.S. states (Alaska, Washington, Idaho, Montana, North Dakota, Minnesota, Michigan, Ohio, Pennsylvania, New York, Vermont, New Hampshire, and Maine) are located along the border.
Document 5: Sumas-Huntingdon Border Crossing: The Sumas-Huntingdon Border Crossing connects the cities of Sumas, Washington and Abbotsford, British Columbia on the Canada–US border. It can be reached from British Columbia Highway 11 on the Canadian side and Washington State Route 9 on the American side. This crossing has been important since this part of the border was established in 1846, but it would be several more decades before settlements were established on both sides of the border. This is because level terrain made this an ideal location for both roadways and railroads to cross the border. Border inspection services were first established here in 1896. The land surrounding the Sumas-Huntingdon border crossing is so flat that it is prone to frequent flooding.
Document 6: Stephen Richard House: The Stephen Richard House is a historic house at 239-241 Elm Street in Southbridge, Massachusetts. It is a 2.5 story wood frame house with two interior chimneys and fairly basic vernacular Federal styling. Its date of construction is unknown: structures with commercial uses appear at its location on maps of the area dated 1855 and 1870. The house's unusual angle with respect to the street (unlike its later neighboring houses), suggests an earlier date of construction. It is first definitively identified on an 1879, owned by Stephen Richard. Richard was a French Canadian who immigrated to Southbridge and established a cutlery making firm in 1862. He is locally notable as the first French Canadian to become a United States citizen, something that he apparently needed to acquire a license of some sort. In his wake a significant number of other immigrants also became citizens.
Document 7: Collins–Valentine line: The Collins–Valentine line, or Valentine–Collins line, is the boundary at approximately 45 degrees north latitude that separates the province of Quebec from the states of New York and Vermont. It was surveyed and marked by survey monuments in 1771–3 by John Collins, surveyor-general of Quebec, and Thomas Valentine, a commissioner appointed by the government of New York.
Document 8: Skinny House (Boston): The Skinny House at 44 Hull Street in the North End of Boston, Massachusetts, USA, is an extremely narrow four-story house reported by the "Boston Globe" as having the "uncontested distinction of being the narrowest house in Boston." According to the executive director of the Boston Landmarks Commission, "In a city where there are many narrow lots, this far exceeds the norm. ... As far as we know, it is the narrowest house in Boston." It was originally built as a spite house.
Document 9: Hyder - Stewart Border Crossing: The Hyder - Stewart Border Crossing connects the communities of Hyder, Alaska and Stewart, British Columbia on the Canada–US border. It can be reached by British Columbia Highway 37A from Stewart and International Street from Hyder. There is no US border inspection station at this crossing, making it the only land border crossing where a person may legally enter the United States without reporting for inspection. The US closed its border station, which was located in the same building as the Boundary Gift Shop, in the 1950s. As a result, all flights leaving the Hyder Seaplane Base to other cities in Alaska are treated as international arrivals, and all passengers, including Hyder residents must be inspected by U.S. Customs and Border Protection officers. Hyder is the easternmost community in Alaska.
Document 10: Haskell Free Library and Opera House: The Haskell Free Library and Opera House (French: Bibliothèque et salle d'opéra Haskell) is a neoclassical building that straddles the international border in Rock Island (now part of Stanstead), Quebec and Derby Line, Vermont. The Opera House opened on June 7, 1904, and was deliberately built on the border between Canada and the United States. It was declared a heritage building by both countries in the 1970s.
Document 11: Tecumseh Opera House: The Tecumseh Opera House, located at 123 S. Third in Tecumseh, Nebraska, is a historic building built in 1880. It is a two-part commercial block building, and has also been known as Seaver Bros. Opera House, as Smith Theatre, as Hahn Opera House, as Spicknall & Goodman Opera House, as Goodman & Canfield Opera House, and as Villars Hall, and it has been denoted NeHBS #J007-53 and OHBIN #ll-29-OI.
Document 12: Warren Opera House Block and Hetherington Block: The Warren Opera House Block and Hetherington Block are historic buildings located in Greenfield, Iowa, United States. They are both 2½-story brick structures. The Opera House block, originally owned by E.E. Warren, is located on the corner and features a corner turret. It housed Warren's dry goods store and a theatre. The adjacent commercial block was originally owned by John J. Heatherington, and is similar in style to the Opera House block. Both buildings feature facades with a tripartite arrangement and center frontispieces that project slightly forward, a broad rock-faced beltcourse that runs above the second floor windows, a narrow metal cornice, and a brick parapet with finials. The Opera House's parapet has a triangular pediment with "Opera House" on a rectangular base, and the Hetherington Block has a similar feature in a simplified form. The buildings were listed together on the National Register of Historic Places in 1979. In 2014 they were included as a contributing property in the Greenfield Public Square Historic District.
Document 13: Northwest Angle: The Northwest Angle, known simply as the Angle by locals, and coextensive with Angle Township, is a part of northern Lake of the Woods County, Minnesota. Except for minor surveying errors, it is the only place in the United States outside Alaska that is north of the 49th parallel, which forms the border between the U.S. and Canada from the Northwest Angle westward to the Strait of Georgia (between the state of Washington and the province of British Columbia). The land area of the Angle is separated from the rest of Minnesota by Lake of the Woods, but shares a land border with Canada. It is one of only six non-island locations in the 48 contiguous states that are practical exclaves of the U.S. It is the northernmost township in Minnesota and contains the northernmost point in the contiguous 48 states. The unincorporated community of Angle Inlet is located in the Northwest Angle.
Document 14: Enosburg Opera House: The Enosburg Opera House is a historic performance venue and meeting space at 123 Depot Street in Enosburg Falls, Vermont. Built in 1892 by the B.J. Kendall Company, a prominent local employer, it has been used since then as a regionally prominent performance venue. It also serves as the site of Enosburg's town meetings. It was listed on the National Register of Historic Places in 1978.
Document 15: Phoenix Opera House Block: The Phoenix Opera House Block is a historic building in Rushville, Illinois. Built in 1882, the building housed commercial businesses on the first floor and an opera house on the second floor. The opera house hosted traveling performers and theater companies as well as local social events. The opera house closed in 1910, as churches began to host the town's social functions and the local movie theater provided entertainment. From 1924 to 1956, the opera house served as a Masonic lodge. The building is one of the few surviving examples of a combined opera house and commercial building.
ground_truth: It was declared a heritage building by both countries in the 1970s.