The mark is called a proof mark, which means at least the barrel, if not the entire gun was built probably either in Belgium or England. Google this ---antique firearms proof marks --- and you will get loads of information. If you can compare the proof mark on your gun with one of the many proof marks you will find in your research, many of your questions will be answered. Regarding who ever is saying your gun has been converted from flint, the lock mechanism has to be inlet into the wood. That means wood was cut away, and I see no evidence of that. I would like who ever is saying that your gun is converted from flint lock, to show me a picture of a back action flint lock that was used on anything.