A gun kit such as this model has a barrel, a frame, and a cylinder. It is designed to be assembled and to shoot. So each piece has to be proofed before it goes into the kit. It is just a matter of safety. That's the way we see it in Europe anyway.
Once each piece has been proofed, it bears the proof markings, including the date code. I don't know if this was mandatory back in the 1960s and 1970s, but it is now.