A total of 73 former workers, including two from Shropshire and one from Staffordshire, were wrongly jailed over financial discrepancies caused by a computer glitch.
Police are now investigating whether a former employee of technology giant Fujitsu gave false evidence during some of these trials.
Tracy Felstead, 39, from Telford, Rubbina Shaheen, 56, from Shrewsbury, and Carl Page, 55, who kept a Post Office in Rugeley, all had their convictions overturned last year after it emerged that faults with the Horizon computer system could have been responsible for accounting shortfalls.
The computer expert, a man in his 60s, has been questioned under caution for the third time as the investigations into perjury allegations continue. A woman in her 60s has also been questioned on two occasions.
The man was used as an expert witness during trials to uphold the Post Offices claim that computer errors could not cause unexplained accounting shortfalls, for which the subpostmasters were blamed.
At the time he was working for Fujitsu, the company which produced the controversial Horizon computer database which has been blamed for creating financial shortfalls.
More than 3,000 former Post Office workers – mainly branch sub-postmasters – were falsely accused of theft or false accounting as a result of the shortfalls.
Miss Felstead, of Bournside Drive, Brookside, was jailed for six months in 2001 after being convicted of stealing £11,500 from the branch where she worked as a counter clerk.
Mrs Shaheen was jailed for 12 months in December 2010 for false accounting after a computer reported a £43,000 shortfall in accounts at Greenfields Post Office in Shrewsbury. Mr Page, who kept the branch in Anson Street, Rugeley, received a two-year sentence in 2007 after being convicted of stealing £94,000.
The Post Office denied the system could have been to blame until 555 former post-office workers launched a group legal action in 2018. The Post Office agreed to an out-of-court settlement after a High Court judge ruled that there was a fault with the computer system.
Mr Justice Fraser also wrote to the director of public prosecutions, Max Hill, saying he believed that untruthful evidence had been given to courts on previous occasions.
Judge Fraser said the company was aware of glitches with the system as early as 1999, but had failed to disclose them.
A statement from the Metropolitan Police said: “A man in his 60s was interviewed under caution again on Tuesday, 26 April 2022.
"No arrests have been made and inquiries continue.”
An independent inquiry into the miscarriage of justice, headed by retired High Court judge Sir Wyn Williams, opened in February.