https://en.m.wikipedia.org/wiki/Shaper#:~:text=In%20machining%2C%20a%20shaper%20is,archetypally)%20linear%20instead%20of%20helical.
Did the previous owner run the receiver both ways on an old Shaper at a machine shop?
Knurling Tool? Pattern is somewhat like a K98 sniper buttplate but different.