Looks like my four Mk10's. (I purchased mine new in 1987 and 1988.) This version is not cut with the channel that the SPEC ring (is that what it's called?) sits in.
I began using two of them for deco bottles for extended range Great Lakes diving in the mid-1990's. The first one was used as is (well, after being cleaned and then lubed with oxygen-compatible grease) on my deeper deco bottle. And the second was converted (using an OEM SP conversion kit) to be an oxygen reg and used on my shallow deco bottle. Both worked fine.
ETA: I never "environmentalized" any of them. And I switched them all to 300 Br DIN (using an OEM SP DIN kit).
ETA2: I used them on OMS (Faber) LP46 cylinders, my very first deco cylinders, pumped to 3,000 psig. No issues whatsoever.
rx7diver
