Big country,
Another potential maker is:
Field John P & Co;14 Upper East Smithfield ,London. Active 1862-1866.
There is also a family connection with the company,
Parker Field & Sons a London gunmaker in business at various locations from 1822-1881.
As recommended by by others ,Photographs of the gun including proof marks, together with details of any street address[ if any!]some times found on the top rib of the barrels provide vital clues to identify the maker and age of a gun.

Roy Hebbes