The numbers are stamped into the transmission case not the engine. The 960 is all about the transmission, which is the only part that is serialized, not the motor. At some point in its history someone put a transmission from a 960 into it. They will swap just fine, it appears to be a transmission from a 56-57 model tractor.
The COMM 6015J casting number is correct for 1964, 172 ci engine, same as I replaced the engine in my mutt 850 with years ago. The engines are NOT serialized, they only have casting numbers for identification. It is a non-sleeved engine used just before Ford switched to the 3 cylinder.