The barrels definitely have London proofs, so English origin. And the loop has indeed been very crudely moved rearward as if to mate up to a different stock. Assume the barrels are from one gun, the remainder from another. The fit of the rib at the rear is normal, this allows the right barrel breech plug ( left on the picture) to be removed first, followed by the left one. The inscription on the top rib is suspect, any British maker associated with the famous name Manton such as Manton and Company would also include the business address and would not normally include the wording London Fine Twist. This is exactly the type of wording used on cheap Belgian knockoffs made for sale outside Britain to gullible settlers worldwide, mostly in North America. The overall original quality shown is very shabby in comparison to Tiriaq's gun, condition not withstanding, it may have been pieced together and sold as a trade gun destined for a native in some far off land.
Shoot it? To start with, note two deep dents or gouges across the right barrel just ahead of where the loop was originally. Not good. If you can get the breech plugs out you can really scrub the bores and then get some idea of condition but if the rust pitting in places on the outside of the gun that is easy to care for is any indication, the inside has been severely neglected. You like your fingers, hands, eyes as they are? This is the perfect definitive wallhanger, complete, cosmetically pleasant, interesting and not safe to use. If you insist that it must go bang, load a stout load, tie 'er down, hide behind a barrier and pull the triggers with a loooong string. I guess at worst you will have destroyed an old dangerous clunker, at best you will be able to say I shot it .....once. Look elsewhere for a regular shooter.