It's taken 6 years of research since he died, but I now have the parts, tools and expertise to fix the 12 dead pistols we have now. One issue is that you need a special small pin spanner to take the regulators apart. I spent considerable time machining a fancy one out of steel, only to discover that the force required to unscrew the adjustment bushing is pretty low. I designed a 3D printed tool with a U-pin of spring steel, and made up a batch:
The problem is a special piston seal they use in the regulator. Instead of an O-ring, they use a "X-Ring", sometimes known as a Quad-Ring. These tend to shred:
Many of the repair parts for the pistol are used in other Hammerli/Walther air guns, and are still available, but not the piston seals. I suspect they ran out of spares a while go due to the high failure rate. I managed to buy a couple complete AP40 regulators, which included the required seals. I measured them, assuming they were going to be some exotic metric X-ring. I discovered they are bog standard "-007" sized ASTM/SAE X-Rings, available from McMaster Carr at $11.21/100.
I've just finished re-sealing seven pistols over the last couple weeks, with 5 more to go...
My one concern is that we had some fail within 2 years of being converted. I don't know if the X-ring is overstressed, or if the seals Larry had were old & tired. The ones I bought are Durometer (hardness) of 70, and I suspect a Durometer 90 seal would hold up better. I haven't found any on-line, but I haven't had time to ask around. The good news is that it only takes about a half an hour to take the regulator apart, clean & lubricate everything, replace the seal and reassemble it. If you take notes of where the adjusted was when you start, the velocity will come up within around +/- 25 fps of the nominal 475 fps.
One of the selling points of using X-seals is they are lower friction than an O-ring. I've been adjusting the pistols with a chronograph, and I've never seen such consistent velocities. They typically run +/- 1 fps, which I suspect is due to the X-Ring seals.
If more details would be useful to anyone, send me a PM. If I was an individual with one of these pistols, I would have abandoned it long ago, but with 21 on of them, it was worth it to sort out. At some point when we can no longer get "in date" cylinders", we will probably sell them off. We'll provide them with the tool & spare seals required to keep them running for another decade or two.