Question: During the 1968 Baltimore Colts season Earl Edwin Morrall filled in for a quarterback that was most valuable player in what years?

Context paragraphs:
History of the Baltimore Colts: The professional American football franchise currently known as the Indianapolis Colts was originally based in Baltimore, Maryland, from 1953 to 1984.  This article chronicles the franchise's history during its time as the Baltimore Colts.  Named in honor of Baltimore's history of horse breeding and racing (including the Preakness Stakes, one of the events in the U.S. Triple Crown championship series) at Pimlico Race Course, this was the second incarnation of the "Baltimore Colts" after the first one played from 1947 to 1950 in the old All America Football Conference for three years and moved to the National Football League (NFL) with a few other AAFC teams after the 1949 merger, but playing only one season.  The 1953-83 Baltimore Colts team played its home games at Memorial Stadium on 33rd Street during its entire tenure in Baltimore before the franchise relocated to Indianapolis, Indiana, in March 1984.

Johnny Unitas: John Constantine Unitas ( ; Lithuanian: "Jonas Konstantinas Jonaitis" ; May 7, 1933 – September 11, 2002), nicknamed "Johnny U" and "The Golden Arm", was an American football player in the National Football League (NFL).  He spent the majority of his career playing for the Baltimore Colts.  He was a record-setting quarterback, and the NFL's most valuable player in 1957, 1959, 1964, and 1967.  For 52 years he held the record for most consecutive games with a touchdown pass (which he set between 1956 and 1960), until quarterback Drew Brees broke his long standing record on October 7, 2012.  Unitas was the prototype of the modern era marquee quarterback with a strong passing game, media fanfare, and widespread popularity.  He has been consistently listed as one of the greatest NFL players of all time.

1975 Baltimore Colts season: The 1975 Baltimore Colts season was the 23rd season for the team in the National Football League.  The Baltimore Colts finished the National Football League's 1975 season with a record of 10 wins and 4 losses, and finished tied for first in the AFC East division with the Miami Dolphins.  However, the Colts won the division tiebreaker with Miami based on a head-to-head sweep (2–0).  The Colts started the season losing four of their first five games, before sweeping their final nine games and narrowly beating out Miami for the division title.

1977 Baltimore Colts season: The 1977 Baltimore Colts season was the 25th season for the team in the National Football League (NFL).  The Colts finished the NFL's 1977 season with a record of 10 wins and 4 losses, and tied for first in the AFC East division with the Miami Dolphins.  However, the Colts finished ahead of Miami based on better conference record (9–3 to Dolphins' 8–4).  This season would mark the final season in which the Colts would make the playoffs as a Baltimore-based franchise (the Colts next appearance in the playoffs came 10 years later in 1987, by which time the team moved to Indianapolis).  Baltimore would not have a team in the playoffs again until the Baltimore Ravens advanced to Super Bowl XXXV, just over 23 years later.

1972 Baltimore Colts season: The 1972 Baltimore Colts season was the 20th season for the team in the National Football League.  The Baltimore Colts finished the National Football League's 1972 season with a record of 5 wins and 9 losses, and finished third in the AFC East.  Carroll Rosenbloom and Robert Irsay, who had recently taken over the Los Angeles Rams, traded ownership of the two franchises, with players and coaching staffs remaining intact.  However, the Colts were getting older and started 1-4 before Coach Don McCafferty was fired.  The Colts would go 4-5 in their final nine games under John Sandusky to finish with a 5-9 record, their first losing mark in 16 years.

1973 Baltimore Colts season: The 1973 Baltimore Colts season was the 21st season for the team in the National Football League.  The Baltimore Colts finished the National Football League's 1973 season with a record of 4 wins and 10 losses, and finished tied for fourth in the AFC East with the New York Jets.  The Colts lost the tiebreaker to the New York based on head-to-head series (0–2).

Earl Morrall: Earl Edwin Morrall (May 17, 1934 – April 25, 2014) was an American football player who was a quarterback in the National Football League (NFL) for twenty-one seasons.  Morrall, who also occasionally punted, played 21 seasons in the National Football League as both a starter and reserve.  In the latter capacity, he became known as one of the greatest backup quarterbacks in NFL history.  During the 1968 Baltimore Colts season, he filled in for an injured Johnny Unitas leading to an NFL championship shutout victory and Super Bowl III, and for the 1972 Miami Dolphins season (both under coach Don Shula) he filled in for an injured Bob Griese leading to Super Bowl VII and the only perfect season in NFL history.  Morrall made Pro Bowl appearances following the 1957 and 1968 seasons.

1970 Baltimore Colts season: The 1970 Baltimore Colts season was the 18th season for the team in the National Football League.  The Baltimore Colts finished the National Football League's 1970 season with a record of 11 wins, 2 losses and 1 tie.  They won the first AFC East title.  The Colts finished their season in Miami with a Super Bowl victory over the Cowboys, their first Super Bowl title and 3rd world championship overall (1958, 1959, and 1970.)

1965 Baltimore Colts season: The 1965 Baltimore Colts season was the 13th season for the team in the National Football League.  The Baltimore Colts finished the National Football League's 1965 season with a record of 10 wins, 3 losses, and 1 tie, tied for first in the Western Conference with the Green Bay Packers.  Although the Packers won both regular season games over the Colts, no tiebreaking system was in place in 1965, and a playoff game was required to determine the Western Conference champion, who would host the Eastern Conference champion Cleveland Browns for the NFL title.

1976 Baltimore Colts season: The 1976 Baltimore Colts season was the 24th season for the team in the National Football League.  The Baltimore Colts finished the National Football League's 1976 season with a record of 11 wins and 3 losses, and finished tied for first in the AFC East division with the New England Patriots.  However, the Colts finished ahead of New England in the AFC East based on a better division record (7–1 to Patriots' 6–2).

Supporting paragraphs: Earl Morrall Johnny Unitas
Answer: 1957, 1959, 1964, and 1967