The "drum & nipple" conversions we see so often are very rarely seen in Europe. As far as I know, it was never used for military purposes so that when you see a European musket converted this way it is almost a certain it was done in the US.
When European military guns are converted they usually have either a bolster breech added or utilize the nipple offset in the top of the barrel. Our Ordnance department referred to the official US Government conversions as using the "Belgian system." This is why you almost never see a pre-1816 musket with that sort of conversion...those guns were sold as surplus before they were converted. Almost all of the later muskets were converted by the Ordnance Department according to the "Belgian System" so when you see a drum on one of those it implies the gun was sold out of service as a flintlock.
Conversions of European guns done in Europe are much less common, largely because the part of the population that hunted or engaged in shooting probably represented a much higher economic level and could afford to buy new guns. for the same reason, we see far more unaltered flintlock guns in European auctions. Exceptions would be something like a well liked Jaeger rifle used by a servant or gamekeeper etc...but even these are usually more elaborate than the simple drum.
There is a good discussion of this in the testimony of John Dent Goodman before a Parliamentary Commission of Inquiry in 1854. Its Appendix 5 in Bailey & Nye "English Gunmakers". Goodman has to explain what a "drum & nipple" is to the commission (All MPs, most of whom I am certain were familiar with sport shooting). He describes the practice as primitive but very popular in the US.
There are probably exceptions and, of course, some American gunmakers used the more elaborate system (I'm thinking of a very nice N. Beyer rifle I know) but in that case, there isn't any doubt its an American gun. I have a French musket Model of the year IX with a drum & nipple conversion that perplexed me until I found an advertisement from a Boston gun dealer ca.1816 announcing the arrival of a new shipment of "French Imperial Muskets", obviously for sale to Massachusetts militiamen required to furnish their own arms and wanting to do so on a budget. It was probably converted in the 1830s.
