17.5 mm = .689" 16.5 mm = .650"
14 bore = .693" 16 Bore = .662"

I shoot .682" pure lead balls in my .69cal. Sporting Rifle with a 10 ounce (.021") denim patch.
I also shoot .675" and .662" balls, pure or hardened lead with 14 ounce (.034") denim patches. All patches for hunting patches are lubricated with pure Neetsfoot Oil(not compound) or a "mink oil" lubricant once sold by a store in the USA.
I suggest this SxS gun is a "Cape Gun" meant for both big game and birds. With a cylinder bore (no choke) barrel, there is no possible reason to want to further spread the shot, imho.

BEAUTIFUL GUN by the way. Such incredible shape. Yes, bayonet mount. Saw a picture/tin plate? of some blokes outside a cabin high in the mountains, hunting camp by the looks of it, wherein one of them had a SxS with a bayonet mounted. This picture would have been taken mid 1800's.