The underside barrel marking is correct as a British Barrel Maker from Rose Brothers, the lock markings are consistent with a typical Tower of 1863. The markings on the upper barrel are puzzling and do not appear as made for the US/CS market due to the lack of gauge markings of 25 or 24. No broad arrow markings for the British army........
The G in a box was indicative of an exporter to the Confederacy, William Grazebrook, of Liverpool, but he was denied additional business after his first contract in 1861 for unscrupulous practices. The marking was typically not on the barrel, but on shipping cases.
Stock markings may be able to help as well, are there any?