Prefix letters on body stamping: when did they start?

[Mike Costigan]

Barney Sharratt's Men and Motors of the Austin has a passing reference to body stampings: Frank Fletcher, who worked at Longbridge for almost fifty years, is quoted as saying: "My first job, at 16 years old in 1924, was making Seven Horse floors in South Works. When you finished you stamped your number on the tunnel to show who had done the job ..."

This suggests that at least for early cars, this was a quality control system,and presumably would not have been either a Body Number or a Car Number.