The sights pretty much says it all. It doesn't matter what the weight is as if it's got sights and no rifling, it was meant to shoot round ball as well as shot. The underside of the tubes should shot the loads recommended.
: Measure the muzzles and do not use a ball larger than those.
; I have a friend who has an English 6 bore muzzleloading double, meant for ball or shot. H&H refurbished it back in the early 80's. Now, it looks new as it was in excellent shape when sent. Holland's re-bored, lapped and polished the tubes to 5 bore. Wil used to shoot trap with it - all 16 pounds of it.