The original IDA-71 DSV is a thing of beauty. It's manufactured from chrome plated brass, and has stainless-steel springs and mica plates for flow control. it is, in fact, a copy of the pre-war Draeger DSVs used in the Lieutenant Lund rebreathers and similar systems. This valve is completely covered elsewhere on the website, most notably on the main IDA-71 page. Go see it in detail if you like, and then return.
The issue on the DSV is that it's designed to accept a full-face mask (shown on the main IDA-71 hose and DSV page linked just above), and not for a standard mouthpiece. What's needed is a round to oval adapter, that's able to be correctly indexed so the mouthpiece is oriented correctly for the diver. Thanks to the magic of CNC milling machines, and somebody who actually knows how to use it (not me!), we have gotten just the thing.
Here's the two piece adapter, the round to oval part, and the retention nut. Not shown is a neoprene gasket to keep the whole thing waterproof.
And here's the result, a perfectly adapted original DSV.
But you *knew* that I'd do something else, didn't you? I have standardized on Draeger Ray hoses and DSV's for all of my rigs, as they are simple, durable, have good flexible hoses, and are cheap. We just need to adapt them to the rig.
What we need to do it to machine new spools to push into the hoses, while reusing the original Russian nuts that screw onto the rig.
Here's the whole shootin' match laid out on the bench. The Ray hoses and DSV, then the grey spools that I machined, with the original Russian spools for the smaller diameter hoses below that, then the nuts, and finally the gaskets. Use the new plastic ones and the hoses fit perfectly. Material is Type II PVC, machined from solid rod.