More trapdoor ?

[Blackrock]

In the butt plate is a swing out rectangular piece of metal. Like a swing open door. There is nothing below this except the wood of the stock. Any answers as to what/why this is like this?

 

[Steve Swanson]

The buttplate is a replacement from a later 1884-88 rifle on an older stock.

 

[Blackrock]

The buttplate is a replacement from a later 1884-88 rifle on an older stock.

Thx.
After obtaining Joe Pyor's book and going through it with the gun in my lap it is clear that this is a cutdown rifle to carbine-like style. Given the 1879 rear sight marked with a "C", the thickness of the metal at the bore, the 1884 breech without the Buffington sight, the 1889 cartouche with a serial number indicating late 1888 manufacture, no saddle ring and no inletting for same, and the carbine style butt plate with no 'tunnels' for the 3 piece cleaning rod. There is a tunnel for the rifle style ramrod with the catch mechanism in the forestock but is obviously too short for the ram rod length. So a rifle cutdown with cobbled parts to appear to be a carbine. I'm OK with that. The trade was fair. I'm told it shoots "decent" and it gives me an inexpensive entry into the trapdoor world.
Down the road I can perhaps obtain a 'real deal'....but for now this is an OK starting point
The Marble Company peep sight and front globe were apparently added at a later date. My laser bore sighting tool says the 1879 rear sight and front globe are spot on but the peep is a little off....might need a remount -- another project for after the leaves come down