Clearly a Field's patent action No. 1927 of 1877, used by many gunmakers

The sidelever locking lever may have been fitted by Field but was unusual except for Holland & Holland and Scott makers

Probably rebuilt to a greater or lesser extent (45-70 a logical U.S.conversion from most of the British .450s)
The Knox?form is shorter on this rifle than some pics I have, due to the rechambering??

My own Fields Patent is a Westley Richards in .450 No. 1 Carbine, equivalent to your .45-75

Very nice rifle, thanks for posting

TH44