Background: Frank Victor Swift (26 December 1913 - 6 February 1958) was an English footballer, who played as a goalkeeper for Manchester City and England. After starting his career with local clubs near his home town of Blackpool, in 1932 he was signed by First Division Manchester City, with whom he played his entire professional career. Swift broke into the Manchester City first team in 1933, taking part in the club's run to the 1934 FA Cup Final, where the club triumphed 2-1 against Portsmouth. Three years later Swift won a League Championship medal, after playing in every match of Manchester City's championship-winning season.
Context: During the Second World War, Swift continued to play regularly for City, making 134 wartime league appearances. He also joined the British Army, and was one several professional footballers to enlist at the Army School of Physical Training as part of a scheme devised by the FA. The school was based near Aldershot F.C. leading Swift to become one of several notable players to guest for the club. Others included Matt Busby, Jimmy Hagan, Joe Mercer, Stan Cullis, Cliff Britton and Tommy Lawton. He also guested for several other clubs during the war including Liverpool. He also played for several representative sides. On 30 October 1943 at Ninian Park, Swift played for a Western Command XI against a Cardiff City XI which featured Lawton and Mercer as guests. The game was a fundraiser for Royal Artillery prisoners of war. In April 1944 in Edinburgh he played for a British Army XI that included Jack Rowley, Leslie Compton, Cullis, Mercer, Hagan and Lawton and against a Royal Air Force XI that included Peter Doherty, Stanley Matthews and Ted Drake. The Army won 4-0. On 9 September 1944 at Windsor Park he played for a Combined Services XI in an 8-4 win against Ireland. This team was in effect a Great Britain XI and featured, among others, Busby, Matthews, Lawton, Mullen, Raich Carter and Stan Mortensen. In the same year he also represented an FA Services XI in games against France and Belgium. In May 1945 Swift also travelled with a British Army XI on a European tour.  In the first season after the Second World War, Manchester City won the Second Division championship to secure their return to the top flight. In the process Swift kept a club record 17 clean sheets in 35 appearances, a mark which was not surpassed until nearly 40 years later, when Alex Williams broke the record by keeping 20 clean sheets in 1985.  By 1949 Swift was 35 years old, but was still performing well enough to maintain his place in the England team. However, he decided that he wanted to retire while still capable of playing at the top level, rather than fading away. To this end he announced that he would retire at the end of the 1948-49 season. He made what he intended to be his last Manchester City appearance against Huddersfield Town on 7 May, which was followed by a parade in his honour organised by the Supporters' Club. However, just before the next season started, Swift's replacement Alec Thurlow fell ill with tuberculosis, and Swift agreed to step in until City found a new goalkeeper. Four further appearances took his career total to 338. Aware of attempts by other clubs to tempt Swift out of retirement, most notably by Manchester United, the Manchester City board continued to hold Swift's playing registration for several years.
Question: when was it surpassed?

Answer:
a mark which was not surpassed until nearly 40 years later, when Alex Williams broke the record by keeping 20 clean sheets in 1985.