Among those who had worked under Welchman in Hut Six is 101-year-old James Thirsk, who attended the launch on Thursday.
Thirsk had initially been an infantryman but had applied for a transfer when a message was circulated for those with suitable qualifications to join the Intelligence Corps as an officer.
His party had been stationed at Beaumanor, where he told us he worked in uniform and didn't receive the suspicion that some other men reported. "If people asked I told them I worked for MI8 communications, which I did," he said.
"I really only have the one memory of Welchman," Thirsk told The Register. "We knew someone called 'Welchman' was in charge of Hut Six, but we never saw him, he didn't mix with us."
"Our party had been moved to Bletchley from Beaumanor in Leicestershire. We dealt with traffic analysis and callsigns – three letters identifying the German stations that were changed every midnight."
"We hadn't known that Enigma had been broken. It was in the middle of the war, Welchman gathered 50 men – we were only men then – and told us about Enigma."
Thirsk has published a personal memoir of his time at Bletchley, comically subtitled An Inmate's Story.
He told us that while his work at Bletchley didn't inform his later professional life as a librarian, his wife, whom he worked alongside on Hut Six's Sixta traffic analysis group, often credited her Bletchley experiences as a core skill in her later academic career.
Joan Thirsk, CBE, was an economic and social historian who worked at the London School of Economics and the University of Oxford. Her obituary in the Telegraph noted that she "was the leading agricultural historian of her generation and had a huge impact on her field in terms of methodology and research."
The exhibition opened following the broadcast of a BBC documentary on Welchman, subtitled Codebreaking's Forgotten Genius. The documentary was extensively recorded at Bletchley Park, and is available on the BBC's iPlayer. ®