Jerry,
The date code should be stamped on the "curved steel plate that supports the rear edge of the saddle" you mentioned. Normally the date is the last two numbers of the year and a letter, i.e. 58 B. That is if the saddle is marked. I have several Brooks saddles in various styles and dates ranging from the early 50's to 1978. Only about half are marked. Some are marked in the middle of the curved support cradle and some on the very end.
Hope this helps.
Regards, Rich Robinson
Jacksonville NC