I have a '54 Manhattan with A/C. It's "aftermarket" but vintage with "ARA Air Conditioned" decals on the wrap-around part of the rear windshield on both sides. And, the car is still - and will remain - 6 volt.
You'll pretty much have to try and find a unit that has been pulled from a car that is functional and put that on from what I can tell. I've checked the sites that offer AC for classics but they all seem to have dropped 6 volt units.
In my case, the compressor clutch is 6 volt, along with the solenoid that runs the clutch, and the fan motor on the inside unit. It would be easy to find 6 volt replacements for the fan motor and the solenoid, but not for the drive clutch in the belt drive.
The Evaporator inside the car has what appears to be a "Vkeep" chrome logo on top of the unit and the compressor is a York unit mounted directly on the original plate that the supercharger was mounted on. When I got the car two years ago it "ran" but not cold. I bought a duracool kit from Wal-Mart (R12 substitute) and recharged it. Took less than a pound so it had not lost all it's gas. Now it blows VERY cold.