Taking what's been said into consideration, it now appears the 'drum' with bottom pan vent for the flint arrangement is a screw-in drum afterall.
Capchee, I now see what you're talking about - the appearnce - I'm thinking the top rib goes all the way to the muzzle however the barrel is actually round all the way to the breech, however a side flat is filed at least on the lock side which stops near the rear sight. Interesting and 'nice'.
I think Dennis' 'conversion' probably works just fine & certainly easier (& safer) than removal of the pan and frizzen, spring, etc, replaced with a drum.