Vintage Double Hose

Came across an early DA Aquamaster with no hookah port in the collection of Phil Nuytten - with his blessing. This one has the 'usd' inside an oval (all in lower case) stamped into the body on each side of the IP chamber. I have seen photos of that but this is the first body I have seen for myself. Phil did not recall noticing the stamp before nor had he seen it on any other units (His collection is quite large so no shocker there). I know this has been addressed in a few of the prior forums and a few of these are probably in the collection of you very readers. What is the current thought on that stamp?

Any feedback appreciated. See Photo.
Dive Safe