Tranny also has date codes stamped along with partial VIN, and the rear end has a code stamped in the middle of the passenger side tube towards the front edge. It will have leters and numbers that will tell you when it was made, where and by whom, as well as the gear ratio it had when it left the factory, and whether it originally had a Posi in it or not.
If it has a number on it then Alan Colvin's Chevrolet by the Numbers will tell you everything there is to know about the marked parts. He wrote five books. One to cover years 1955-1959, another documents 1960-64, the next covers 1965-69, then he has one that covers 1970-75. His last book is all about his true passion which is documenting Corvettes. He is the acknowledged master of decoding and worked with GM with access to a lot of their files before they were destroyed.