Need help with original ( I think )1875 Rem. pistol

[Leonardallen]

Have an old pistol that appears to be an original 75 Rem., It was found in a barn in NW Indiana about 25 years ago.  I have been reading all the info that Pards have sent me but I just can't wrap my head around it.  It is so rusted and pitted that I can't find any markings on it.  It has the hammer style and cylinder ratchet measurement, as well as the pinch site of the first model.  The cylinder openings measure .458 - .460 on loading end with .454 - .455 on the barrel end.  Barrel slugs out to be .417 - .421.    Any thoughts?  Please!