The screw on my 1985 proofed GPR has a #10-32 UNF thread. It measures 1.695 from the top of the semi-domed head or about 1.650 from the place where the angular surface of the cone meets the outside of the head. (This would be about the length of the screw if it was a flathead screw.)
The bolt I was looking for the size of was #11a in the parts diagram Boomerang posted. I determined that it's a M5 size, and I was able to pick one up at my local hardware store. Just had to cut it to length and trim the head diameter down a bit to fit flush in the tang.
Dixon's Muzzleloading Shop in Kempton, PA., is an authorized factory repair/warranty shop for Lyman rifles; give them a call, they know it all! Most ML guys are aware of their annual July muzzleloading fair, last full weekend in July.