The threads turned out to be M2.5. Unfortunately, the top two turns in the gun body are loose - maybe stripped? Not sure how that could have happened...
I found a site that specializes in metric screws and ordered a bunch of wafer-headed M2.5's in a bunch of lengths. I also picked up some toothed washers in various designs. It's possible that the tension from the springs, plus the washer biting into the screw head, will be enough to keep it from moving. I'll probably add some green Loc Tite once I have it zeroed.
Failing that... I also have a Dan Wesson elevation screw on the way. The head is almost exactly the same size, but it's 6-48 threaded. I can have the gunsmith re-tap the hole and I'll definitely have enough threads to get the zero.