This might be of interest.

Prior to sometime in the 1960s the inside of leather side flaps were stamped with a date code. The stamp denoted the month and year of manufacture. Mrs. O'Donnell* writes: "Unfortunately, the "code" book is now lost so we cannot decipher the actual codes used at that time. It is, sadly, not quite as simple as A=January, B=February etc. This 'flap' stamping changed to the frame stamping around the 1960's although there was an 'overlap' period where both the leather and the frame were stamped."
The metal frames of the saddles were stamped with the date of manufacture from sometime in the 1960s to about 1988.
The current method of stamping Brooks saddles is a colored stamp on the underside of the saddle, with a number for the year surrounding a letter for the month.
(*The above Brooks information was provided in an e-mail from Mrs. E. O'Donnell in the Sales Department of Sturmey-Archer, when that company owned Brooks. Mrs. O'Donnell was writing to Robert Williams in response to his query about date codes. Our thanks to these two individuals and to Larry Osborn, who secured a copy of the original e-mail.)