Bannerman 45-70 "Bitsa" rifle
I had one very similar. The stock was/is a musket stock with the corners of the flats rounded off to make it resemble a 45-70 stock. Remember that these companies were taking obsolete/scrap parts and making rifles to sell to the public. In many cases , they were trying to make the public believe that these "bitsa" rifles were newer models. Mine had 1884 stamped on the lock plate. The barrel channel had several layers of brown paper to take up the difference between the 50-70 and 45-70 barrels. Also , the rear sight was from some other type of rifle as was the cleaning rod.
That looks like an 1892 / 1894 krag rear sight. Go have a look
Thats surely a Bannerman parts gun. The lockplate was NOT made by SA.
Have a look at the 1884 trapdoor I have in the "For Sale" board for a completely correct trapdoor :D
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Trapdoor made of condemned parts
As previously stated by others, you have a TD that was assembled around to the turn of the cnetury for sale as a "near Springfield". The lock was condemned and sold for scrap. Here is a picture of another identical one that shows the C (for condemned) stamped on the back of it.