OK. Since the barrel shows shallow-groove rifling (possibly 3-groove?) I stand corrected. The caliber should measure about .69 (18mm). The French and Belgium muskets of the period were very similar. Parts coming from the same arsenals.
The latter photo showing the bolster seemed to look very different than the earlier photos. I guess that was because I was looking at it with the barrel out of the stock versus in the stock. And with the additional information on the barrel being shallow rifled, I would disgard my earlier theory of the barrel, and agree with you. The barrel was designed to shoot the .69 minnie ball of the period.
So, it appears the entire service musket was utilized and re-stocked (possibly due to a broken stock?) somewhere to suit Ottoman tastes. Possibly at one of the many gun making centers in the Balkans (?). The decoration and engraving seem to be done in a generic fashion. So it's difficult to tell wheather it was made for a specific individual, or just re-stocked in a shop for resale to anyone interested. This re-use of parts was very common throughout the Empire. The gun could have been assembled in India, Afghanistan, or any number of different locations. It's almost impossible to tell for sure. And most of these guns were never marked as to their final maker. But the utilization of Europen parts would likely have put a percieved increase in value to the purchaser. This re-use of parts was very common throughout the Empire.
Again, it's a very neat gun I would not mind having in my collection.
Rick