Re: Is this some kinda military-issue tank ?

[lotec]

Correct, my understanding is they came with the military RDs. They are pretty similar to the 1953 "square" tanks but have that large rim for stacking – they weigh a lot more, but if that’s from thicker steel or the addition of the rim I can’t say. The housing, cap and diaphragm appear to be the same as the consumer motors, though I have not had mine apart to see if there are internal differences. I don’t recall any markings on the bottom but will dust mine off to see.