FIRST: WELCOME HOME, Brother-in-Arms.
I'm away from my "book storage site" (at our family farm in NE TX), so I cannot give you an exact reference, but the term "cape-gun" (one rifle/one smoothbore) appears in an English language book of circa 1840 & states that those weapons were FIRST intended for "The Cape Trade" (Union of South Africa) & that those arms were "becoming commonplace" among "gentleman travelers" & serving (UK) officers, who are enroute/assigned to South African postings or to India.
How early were the "capeguns" made? - I simply don't know that answer but it was PROBABLY some English/Irish gunmaker who simply copied the German rifle/shotgun combination weapon & "named" what he made.
(The memoirs of Lord Robert S. Baden Powell describe a similar 14 gauge "combination gun" that he took to SA, when he was a LT in 1876-78.)
There ARE some European flinters that are rifled/smooth.
SORRY, I just don't know more than that.
yours, satx