My take is that your gun was made in Birmingham England based on the proof marks. It would appear to be a modest gun sold to retailers very likely in North America. It might well be a decent shooter, but should be properly checked out before being fired.
London Twist was commonly inscribed on the barrel rib of these guns, it means laminated steel construction, which was the standard on the 19th century. The Manton on he lock is roughly equivalent to Pontiac putting Le Mans on their cars. There were two revered London gunsmiths named Manton, the brothers Joseph and John, who mostly competed with each other, although they did cooperate on a joint venture to market guns in India. It was common for other smiths in England and Belgium to inscribe Manton or J. Manton on their guns to sound impressive.
