Oh fer crying out loud, yer right--it's brass--but still!
I've thought about how cooky-tins might work a few times. I've seen a few tutorials on Ewetoob on how to cast with aluminum and brass by making your own crucible etc. out of readily available materials. In the early 90's I was doing lost-wax bronze pieces in college: that was fun!
Check this eBay listed reg out. I'm not sure if it's homemade. The only info on it that the seller knows is that it was a friend's father's, and he was in the Navy in the 50s and 60s (which probably has no bearing on its origin). It looks like it is a second stage in the can and has a remote first stage. There is a pressure gauge! It doesn't look like an oxygen diluter conversion.
First stage: Valve and ??: Interesting! It reminds me of the Soviet rigs like the AVM-3 setups.