Spanish Gun Question 18th & early 19th Cen.

[Marcus den toom]

If i where to take a guess, it would be a hole drilled from the butstock where the butplate covers the wood (and also the hole).
Also, the tang could be attached by a normal screw instead of a bolt that is going completly trough the wrist of the stock (and the trigger plate idem dito).

I never owned such a piece so.. hard to tell from fact from my point