Ah! Thanks! That's it. I didn't have any snap caps or anything to measure the headspace with but a 357 cartridge only fits in one cylinder and a 9mm only fits in the other. And upon closer examination, the chambers of the 9mm cylinder are not machined the full diameter all the way through.
And yeah, Ken, the last 3 digits of the serial # and a 'G' are etched on the rear face of the cylinder. So it appears to have been factory matched up for correct timing.