Interesting Combo gun and quite early.
Thomas Barnsley was born 1841 in Birmingham, and as an adult emigrated to the Orange Free State (South Africa) in 1874. Principally had shops in Bloemfontein, Pretoria and Port Elizabeth.
The gun was almost assuredly made in Birmingham and likely by Philip Webley for Barnsley.
When you take delivery and can get pictures of the proof marks, and any other marks, that would possibly be useful.

Those multitude of leaf sights are seemingly typical of guns made for the South Africa trade. I have read that it was not uncommon back in the day, if unable to approach closer, that one would use the "long range" leaf (or sometimes a fitted ladder sight) and blaze off at a herd. The .577/.450 MH bullet will certainly kill plains game at great distances……..Wounding, and later hunter ethics, were apparently of little concern.

ADDED: the Grahamstown barrel address would date the gun to 1877 or later. Barnsley only opened his own business in Grahamstown in 1877, prior to that he worked for Grainger & Sons. All of the Thomas Barnsley shops were closed by 1885-1887.
- Mike