Question: What is the official designation of the building visible from the Morpeth Arms in a manner enabling spying?

Context paragraphs:
Comet Holmes: Comet Holmes (official designation: 17P/Holmes) is a periodic comet in the Solar System, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892.  Although normally a very faint object, Holmes became notable during its October 2007 return when it temporarily brightened by a factor of about half a million, in what was the largest known outburst by a comet, and became visible to the naked eye.  It also briefly became the largest object in the Solar System, as its coma (the thin dissipating dust ball around the comet) expanded to a diameter greater than that of the Sun (although its mass remained minuscule).

The Forum at 343 East 74th Street: The Forum at 343 East 74th Street is a 25-story residential condop building located on the Upper East Side in New York City.  The building's location, midblock between First and Second Avenue on 74th Street makes the building visible from many parts of the Upper East Side.

Morpeth Arms: The Morpeth Arms is a public house at 58 Millbank, in the Pimlico district of London.  It was built in 1845 to refresh prison warders serving at the Millbank Penitentiary.  It now contains a Spying Room which provides a good view of the headquarters of the Secret Intelligence Service across the river.  The building is listed as Grade II and it is now part of the Young's estate.

Morpeth Castle: Morpeth Castle is a Scheduled Ancient Monument and a Grade I listed building at Morpeth, Northumberland, in northeast England.  It has been restored by the Landmark Trust and is now available as a holiday rental home.

Royal Centre (Halifax): Royal Centre in Halifax, Nova Scotia, Canada, is a 13 storey office tower in the heart of Halifax's financial district.  It was built in 1960 and was the regional head office of the Royal Bank of Canada (until it moved to the RBC Waterside Centre in 2014), located on 5161 George Street between Granville and Hollis Streets across from the Provincial Legislature Building.  This building does not violate the bylaw banning the construction of any building visible from inside Citadel Hill (Fort George).

Morpeth Chantry: Morpeth Chantry also known as All Saints Chantry is a Grade I listed building situated adjacent to the site of the ancient bridge across the River Wansbeck at Morpeth, Northumberland.

Comet Lulin: Comet Lulin (official designation C/2007 N3 (Lulin), Traditional Chinese:鹿林彗星) is a non-periodic comet.  It was discovered by Ye Quanzhi and Lin Chi-Sheng from Lulin Observatory.  It peaked in brightness and arrived at perigee for observers on Earth on February 24, 2009, at magnitude +5, and at 0.411 AU from Earth.  The comet was near conjunction with Saturn on February 23, and passed near Regulus in the constellation of Leo on February 26 and 27, 2009.  It was expected to pass near Comet Cardinal on May 12, 2009.  The comet became visible to the naked eye from dark-sky sites around February 7.  It passed near the double star Zubenelgenubi on February 6, near Spica on February 15 and 16, near Gamma Virginis on February 19 and near the star cluster M44 on March 5 and 6.  It also passed near the planetary nebula NGC 2392 on March 14, and near the double star Wasat around March 17.  According to NASA, Comet Lulin's green color comes from a combination of gases that make up its local atmosphere, primarily diatomic carbon, which appears as a green glow when illuminated by sunlight in the vacuum of space.  When SWIFT observed comet Lulin on 28 January 2009, the comet was shedding nearly 800 USgal of water each second.  Comet Lulin was methanol-rich.

Škorpion: The Škorpion vz.  61 is a Czechoslovak 7.65 mm machine pistol developed in 1959 by Miroslav Rybář (1924–1970) and produced under the official designation "Samopal vzor 61" ("submachine gun model 1961") by the Česká zbrojovka arms factory in Uherský Brod from 1961 to 1979.

SIS Building: The SIS Building or MI6 Building at Vauxhall Cross houses the headquarters of the Secret Intelligence Service (SIS, MI6), the United Kingdom's foreign intelligence agency.  It is located at 85 Albert Embankment in Vauxhall, a south western part of central London, on the bank of the River Thames beside Vauxhall Bridge.  The building has been the headquarters of the SIS since 1994.

Great Comet of 1744: The Great Comet of 1744, whose official designation is C/1743 X1, and which is also known as Comet de Chéseaux or Comet Klinkenberg-Chéseaux, was a spectacular comet that was observed during 1743 and 1744.  It was discovered independently in late November 1743 by Jan de Munck, in the second week of December by Dirk Klinkenberg, and, four days later, by Jean-Philippe de Chéseaux.  It became visible with the naked eye for several months in 1744 and displayed dramatic and unusual effects in the sky.  Its absolute magnitude — or intrinsic brightness — of 0.5 was the sixth highest in recorded history.  Its apparent magnitude may have reached as high as -7, leading it to be classified among what are called the "Great Comets".  This comet is noted especially for developing a 'fan' of six tails after reaching its perihelion.

Supporting paragraphs: Morpeth Arms SIS Building
Answer: SIS Building or MI6 Building