I've been reading about use of the Brown Bess variants during the Civil War.
Some Virginia units had flintlock and percussion "English" smoothbores.
Some Louisiana volunteers had "Bess" muskets, certainly conversions, not sure about flintlocks.
The Alabama guards at Andersonville were armed with the flintlock Brown Bess.
Of his troops guarding Columbus, Missouri, Grant writes November 21st, 1861:
The condition of this command is bad in every particular except discipline In this latter I think they will compare favorably with almost any volunteers. There is great deficiency in transportation, I have no ambulances. The clothing received has been almost universally of an inferior quality and deficient in quantity. The arms in the hands of the men are mostly the old flint lock repaired, the "Tower" musket, and others of still more inferior quality. My cavalry force are none of them properly armed, the best being deficient in sword belts and having the old pattern carbines. Eight companies are entirely without arms of any description. The Quartermaster's Department has been carried on here with so little funds that Government credit has become exhausted. I would urgently recommend that relief in this particular be afforded at as early a day as practicable.
- U.S. GRANT, Brigadier General
Col. Heiman's 10th Tennessee in 1862 were armed with:
To oppose this force General Tilghman had less than four thousand men - mostly raw regiments armed with shotguns and hunting rifles; in fact the best equipped regiment of his command, the 10th Tennessee was armed with old flint lock "Tower of London" muskets that had "done the state some service" in the war of 1812.
- Fort Henry and Fort Donelson Campaigns, February, 1862: Source Book. The General Service Schools, Fort Leavenworth, Kansas, 1923, p.17, 497.
Any flintlock Bess' obtained from overseas were probably post-1797 3rd model, India pattern guns. BUT the Virginia contract guns accepted in 1799 (iirc) have features of the 2nd model, Short Land Pattern guns e.g. furniture, barrel length. Additionally, the weapons captured during the war of 1812 (or possibly earlier) could also be of the Short Land Pattern as these remained in service longer overseas, and more generally remained in concurrent use into the early 1800s.