I got the DIN adapter in the mail from Ukraine. I put it on and then the DIN/yoke adapter I got on the Kaiman. This is not practical to dive with (and may be unusable) as the stack pushes the regulator far away from the valve. I could test the reg, though. It is surprisingly good! I need a DIN tank valve now to pool test.
It looks like they made these from a thermostat pulled out of a 1950s Lada.
It seems to be a simple spring loaded plug and guide fitted into the hose connector which will only work on the exhaust side. It's a bit less simple than a mushroom valve, but simple in any case. I wonder if it can be turned around inside the hose connector? I'll see when they arrive!