Hi, That sounds like a 1920's era gun then, the butt trap on these is a wooden oval cover secured by two small screws with a convex oval spring under it to help release, the purpose is to allow access to the stock retaining bolt, If you shine a light you will see a slotted head down the 'ole
Gun below is a 1919-39 era one (they did change a bit but this is showing all the general shapes of the bits etc.)
If it has a full size metal cover to the butt then it is an earlier stock, most parts interchange between eras so can get confusing when stuff is replaced
Also used for hiding messages, or putting weights in etc. etc., but not really designed to be taken on and off as can break if levered out without care.
ATB, Ed