I have never seen any evidence of stain or finish in the box cavity, bottom of lid or dovetails on original guns. The spring is driven in a slot like a nail. Like Lucky, I often use a 16p nail.
In fact, I have never seen any evidence of stain or fish on inside surfaces of old guns....under buttplates, sideplates, triggerguards, barrel channels, none of those places had finish on them on old guns.