John: it is certainly a J. Manton Birmingham, not a Belgian knock-off. Hopefully one of our British correspondents, or Bro. Larry, can confirm the proof marks.
Without knowing the bore measurements ie. that it has not been honed since proving, no estimate of value can be made.
The 70mm chambers are very likely not original, but a modification prior to re-proof.
Wish we could know the history of the barrels from c. 1875 to arrival in the U.S.
1875 is early in the top-lever opener era.
https://www.vintageguns.co.uk/articles/462-2/ 
