Sorry, my KP and none of all the Sheridans I've worked on are like so.
Or I'm missing what you're telling me because english isn't my native language, and it's a bit hard sometimes to stay focus and fully understand everything at 100%.
But I'll look again tomorrow (it's 2am here).
EDIT: is this screw the one with the nut which is used to settle the stock to the tubes?
EDIT2: or you did grab some guns which were already drilled and tapped for. Okay I'd better gonna sleep now... if I can.