These three pin metal cans were very common, especially the LM340 5V regulator which was, of course, a linear regulator. Today, you are more likely to see a 7805 in a TO-220 case or something ...

Sure, it’s probably not the most secure lock on the planet, and the 5V linear regulator is held on to the relay board with hot glue, but this is an exceptionally well-documented project ...