Standard would be 10x32 at .75 inch spacing between the two holes.
Most modern bottomline asas and drops use this spacing. This frame
became the standard I would guess after WGP's Sniper two and then
Autocockers. AGD (made by WGP) also used these same frames (although
slight milled out for the sear on Automags.
Non standard would be 10x32 at 1.25 inch spacing and would apply to
older pumps specifically grey ghosts and other lapco guns. Carters
also use this spacing.
The other type of spacing would be spyder spacing which is the two
diagonal holes which are a smaller outer diameter than 10x32 screws.