I have bought five Rondinax 60 tanks over the past year or so. Excluding the relatively modern plastic version, I was surprised to find that some parts of the bakelite tanks were not readily interchangeable. Even a knob from one tank would not fit the others. Some lids were very tight on some bodies, and so on. So the tactic of buying several broken tanks to build up a perfect one is fraught with difficulty. Presumably, the various marks impressed on the underside of parts indicate which set of tooling was being used for a particular tank. How many tools were made and whether more than one set was in use at any one time I do not know.