An underfolding stock requires a big ol' hole through both sides of the receiver, plus a few smaller rivet holes that are not in the same location as any of the rivets used to secure the fixed stock trunnion. Not a trivial change to make. Besides, underfolding AK stocks suck to actually shoot with. If you really want a more compact AK, try a Romanian or German wire sidefolder first - those will bolt right on to your existing rear trunnion.