It sounds like a vintage 40X receiver with the clip slot, bullet nose cut and possibly a thumb cut (need to see pics). The thin bolt handle is early and quite desirable. These were used on a few different rifles, I've seen them used on some Army AMU shooting team rifles, USMC "Space Guns" (target rifle with McMillan prone stock) and even a few M40A1 rifles in the mid 1990's. While technically not correct for an A1 build, a few slipped through the cracks, but that was very quickly rectified when the 2112's realized it.
5 of those receivers were returned to Remington and exchanged for 700 marked receivers (the Corps also retained a few to use as target rifle receivers). Those 5 returned receivers were then purchased by Dan Ross and he ended up selling them years later. I've owned 2 of the 5 USMC returned 40X receivers, I sold one of them, the other is on a very, very early PWS A3 build (dated February 2001, 10 whole months befire the first batch of A3's were build for sniper platoons in the fleet).
None of the 40X receivers made it to A3 rifles, this is just what Dan wanted PWS to build him over 20 years ago on that receiver. The other 4 40X return receivers were also build into A3's at Dan's request. These 5 receivers now sell for stupid amounts of money, since provenance paperwork is included with them (I have copies for all 5 receivers).
TL/DR: build a target rifle or a somewhat incorrect M40A1.
Hope this helps.
