Check the SCSW for the model, but it was a big bore top break that was sold until 1913 or thereabouts yet had all the frames made before
1-1-1899 that Muley Gil is likely thinking of. I've seen them on the auction sites a time or two.
Any gun that doesn't used fixed ammunition - in-line muzzle loaders, Ruger Old Army revolvers, flintlocks, what not - is considered in the same category under the GCA '68 even if it was made last week. Since the ATF considers conversion cylinders for cap and ball revolvers (which allow them to accept fixed ammunition) to be replacement parts, it's quite feasible to order a cap and ball revolver and a conversion cylinder from Midway, mate the two, and have a "new" cartridge arm.
Energy weapons also aren't directly covered under the GCA '68, though they are regulated by the FDA (in the case of lasers). Thus a spiffy new electromagnetic rail gun would be considered the same as a non fixed ammunition using flintlock.
Flamethrowers are also non regulated by the feds, being considered essentially the same as airguns.