There was a Manton shop in London around the time your gun was made. They sold Birmingham guns of good quality. According the only book I have that notes this shop the gun should have the London address of 6 Dover St on the rib.
That was the company of Joe's elder brother John and his son or sons, marked, accordingly, "J. Manton and Son", rather than "Co". There was also "Manton and Co, Calcutta", started by Joe Manton and his son Frederick in 1820, which also had a London office around the late XIX century, but they (no relation to either Joe's or John's heirs by 1870s) never had a J and always signed "Calcutta" or "London and Calcutta". Lots of confusing stuff, that Manton business
