When I was looking at the original 1872 Open Top at the shop, it confirmed my thoughts that it would be easy to convert one back in the day to .44 Colt.
Consider there were .44 Russian open tops, and their cylinder was the same diameter as the .44 Henry ones. The rim size (which is a factor in chambering) is almost identical between the .44 Russian an Henry (see chart below) and 44 Colt is smaller than .44 Russian.
The Cylinder would need chambers only opened up .015". The bore on the Open Top is .446" and Colt is .450". Of that there is only a small bump just above the heel on the .44 Colt that is that diameter. The lead would easily flow back into the .431" heel area of the bullet.
As for firing pin, I checked the original. The side mounted firing pin could be replace with an angled down and over one with a point on it to hit primers. The notch in the frame/recoil could easily be opened up angling toward the center of the cartridge instead of the side.
The conversion would make sense if someone back then had an open top, but wanted the more reliable centerfire Colt. Considering the work of many frontier gunsmiths on private conversions and long cylinder conversions, it would be an easy job, even with the tools and technology of the time.
Cartridge bore case rim
44 henry .446 .441 .518
44 Colt .450 .456 .483
44 Russian .430 .457 .515