Infantry was originally armed with edged or pole weapons. Gradually small hand cannons were introduced. These crude weapons evolved into matchlocks and arquebus and then wheel-locks. Gradually, these were phased out in favor a the flintlock and all infantry carried smoothbore guns became known as muskets. As such, muskets became the weapon of choice for the infantry for about two centuries. Even when its reign came to an end with the adoption of the minie rifle (around 1851), it was still used as late as the American Civil War (1861-65). Muskets have in common a mechanical sparking device (doglock, flintlock, percussion lock), a long, smooth bore barrel and a wood stock. I believe Vauban formalized the plug bayonets and over time, these were phased out in favor of socket bayonets. The latter gave the infantryman the ability to continue firing as well as the ability to defend himself against cavalry or a ready shock weapon to use at close quarters.
Musket patterns varied with every nation and within every nation, between each generation of soldier. The Brown Bess may have been in service for over 100 years, but it was subject to modernization including changes in stock design, trigger guard, buttplate and sideplate (to use less brass), barrel length (shorter with the first ones being 46" long and the short, 39" India pattern adopted by the time of the Napoleonic era), ramrod (from wood in the First Model to iron during the French & Indian War.
If you get a chance, remove the buttplate or lock mechanism from the stock. Examine the patina and inspect the lock mortise for soundness and fit. You don't want to buy something that is rotted beneath the stock.