Thanks for the additional barrel photos. OK. Octagon fading to round. The most common style. The most common barrel lengths on these "Trade" guns are usually 36" to 42". The longer ones such as your's are more rare. And more desired among collectors.
I'm wondering, even at the turn of the 20th Century, if all gun barrels wheather muzzle loading or modern (for the time), old or new, had to be proof-marked ? Even for civilian use in the Kingdom ? Possibly your barrel is older and re-stamped latter ? But it doesn't look that way. Hmmmm. Don't know.
One other thing I do notice. The wood, at the inletting of the lock mortise looks old. But not old enough to be first half of the 19th Century.
The lock looks to be of good, plain trade gun quality of the original period. As well as the style of iron butt plate and trigger guard. I can't imagine their would be enough of these locks/parts surplus to build some type of production for African trade guns.
So, at this moment, my best guess is that this gun was built sometime after 1914 for civilan sporting use, utilizing an older lock, butt plate, and trigger guard, with a later barrel.
In any case, it's sure an interesting piece.