I mistakenly bought some reproduction Burnside cartridges, approximately 20-something years ago. A clue that mine are repro:
The inside of a civil war Burnside cartridge's lip has a wide "valley" which corresponds to the wide "ridge" on the lip's exterior. My old Repro has the exterior ridge but no valley on the lip's inside. In other words, my Repro cartridge's interior looks like the interior of a plain-walled pipe, or the interior of a Spencer cartridge.
Regards,
Pete