The sliding door mechanism looks like the one on an 1873 rifle that I just repaired.
Except this one was a crescent type(rifle) buttplate.
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From what I have been able to find the rifle was pointy and the carbine rounded at the bottom
It looks too round for a '73. Maybe an 1895 SRC?
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Found one that sold (complete like this one) and was identified as being for the 1873 Carbine. Found one listed on evilbay without the sliding door also identified as being for the 73 Carbine
BP | Bronze Point | IMR | Improved Military Rifle | PTD | Pointed |
BR | Bench Rest | M | Magnum | RN | Round Nose |
BT | Boat Tail | PL | Power-Lokt | SP | Soft Point |
C | Compressed Charge | PR | Primer | SPCL | Soft Point "Core-Lokt" |
HP | Hollow Point | PSPCL | Pointed Soft Point "Core Lokt" | C.O.L. | Cartridge Overall Length |
PSP | Pointed Soft Point | Spz | Spitzer Point | SBT | Spitzer Boat Tail |
LRN | Lead Round Nose | LWC | Lead Wad Cutter | LSWC | Lead Semi Wad Cutter |
GC | Gas Check |