I believe there is one more number in front of your 1201D.
Those numbers are the Model number of the tractor that this transmition was originally installed in.
There is a second number that is further out on that casting, but it may be ate off from battery acid (common on diesels of that era). That would be the serial number.
If that extra number is a 4, you have a 63 or 64 Ford 4000 Diesel.
I am guessing that you will find a 2 there as the rear end of that tractor looks like a 2000.