Check this out. I stumbled upon it after I discovered my father had written down the serial # for my GT12. The page works for many pre-'75 Jacobsen products. Here's the link:
Just choose your equipment, enter the model number, and then the "item" number.
In my case, GT garden tractor, then 53215, and lastly 9374. This confirmed that my tractor is in fact a 1973 model, in keeping with the 1973 date code on the Kohler engine.
Hope this helps a few people...