for a stamped mak-90, you need to install 5 usa made parts to legally run a pistol grip configuration.
the std. mak t-hole buttsock os mounted with the two strews in the rear trunnion (one thru the tang, one inside the removed action cover) and the pistol grip screw is used as a third attachment point.
note: you will need a longer pistol grip screw to affix a std. length pistol grip with!
say you toss in the 3-piece redstararms trigger group, a usa pistol grip from k-var and perhaps k-var's usa made polymer buttstock and handguards...you are 6 usa parts installed. that's one more than legally required.
the chinese receiver is thicker walled than euro receivers, and the opening is 1mm (.040") more narrow (from side to side) than euro receivers. if you try installing a euro buttstock, it must be thinned down .040" thru the tang area...which inserts into the receiver. other than that mod, euro buttstocks fit right up to the mak.
the lower handguard tang also requires this same thinning ti fit inside the nose of the receiver. the upper handguard may require slight trimming/fitting, also. the chinese ak's seem to be just a little off the euro pattern in a few dimensions.