Dan,
Assuming that you are referring to that which is stamped into the underside of the cantle plate, it would be a manufacturing date code indicating that the particular saddle in question originates from the first (...i.e. "A") production run of the calendar year 1975 (...i.e "75). Although I am not absolutely certain, my vague recollection is that the letter code preface itself incremented by fortnight (...i.e. a 14 day span of time). If so, then your saddle would have been built up during the first fourteen days of January, 1975.
Perhaps Tony Colgrave might be able to "chime in" with a bit more information....
Robert "saddle tattle" Broderick
...the "Chronically Cloudy Clime" of Oregon
Portland, USA
> From: danevansdesign@yahoo.fr
> To: Classicrendezvous@bikelist.org
> Date: Mon, 29 Mar 2010 23:54:27 +0100
> Subject: [CR] brooks saddle
>
> hello
>
> what does A75 mean on my brooks professional saddle?
>
> any help much appreciated.
>
> dan evans
> aquitaine
> france