Then what you have is an 1888 Model design. The 1888 was made from 1888-1893. If memory serves me correctly, I believe it was the last Black Powder version of the Trapdoor design by Springfield. The next Military design by Springfield was the M1903.

Whats the serial number and how is it marked on the Breech Block. If it is marked "US MODEL 1884" and has a serial number in the 500000 then it is a common 1873-1888 model. If it is actually marked "US MODEL 1888" then get ready for a windfall because those are exceedingly rare finds.