On the long butt plate return there are at least three methods of securing that tang that come to mind.
First, the hook with a corresponding staple set in the comb wood to receive it beneath the tang.
Second, a lug like a trigger guard has, to receive a lateral pin, a la Brown Bess.
Third, a small screw toward the muzzle most end. If the tang is inlet with a little tension built into it, the big butt-plate screw at the heel puts enough pressure forward to keep it in place.