I'll be honest, I'm not the best at identifying specific makes and models of firearms, so you might want to go ahead and try asking r/guns or r/whatisthisthing if you want to learn more. Be sure to include close-ups of any proof marks or other icons you can find. The overall weight of the weapon and the inside diameter of the bore would be helpful too.

However, I suspect that exact identification will be pretty difficult anyways. Based on the length and decorations it's most likely some sort of fowling piece; a sporting weapon usually loaded with small pellets and used for hunting waterfowl, however its length made it pretty accurate when loaded with a single, tight-fitting ball as well. Fowling pieces were typically not mass produced, in Europe being essentially a luxury item for the well-off, and this one in particular seems to have been modified quite a bit over the years.

I will also add that the weapon you have technically isn't a flintlock, it's a snaphaunce (the wikipedia page includes a really good gif showing it's operation). The true flintlock which became the standard lock mechanism towards the end of the 17th century includes a "half-cocked" position for loading and the frizzen attached directly to the pan. The Snaphaunce was relatively expensive and more complicated so it was generally limited to cavalry carbines and private hunting weapons in Europe while ordinary soldiers continued to use matchlocks until they were replace by the flintlock. In the north American colonies of the early 17th century however snaphaunces and other proto-flintlocks quickly became a must have since were more reliable, more convenient, and didn't require tons of match cord like matchlock firearms did.

If it is a 300+ plus year old weapon you might want to try and get in contact with a museum somewhere.

I am not sure a redirection to another sub is acceptable here, and I apologize in advance if it isn't, but you may have success asking in /r/guns. They're generally pretty knowledgeable at IDing old weapons.