Casting numbers?

Where would the casting numbers be on Th400 and 350's?
I have three BOP Th400's (long-tailed) and a BOP Th350 (short) and trying to figure out the years if at all possible.

Is there a site other than identification

Wayne S

 

For the early 70s the year is engraved into the metal tag on the passnger side.

 

Ahhhh I see...I will have to start in the morning, it's finally raining here (or at least rumbling like the bottoms going to drop out)

Thank you
Wayne S