Ummm. I am going to leave the "balanced out to a pair of stereo headphones" question to someone with a more delicate touch. And I might even go so far as to suggest a call to Comtek when they open in the morning regarding the equipment you own. But here's what I know...
The M72 is a fixed frequency mono transmitter. The frequency is stamped at the bottom of the transmitter, typically. Comtek built these transmitters in several frequency ranges. For example, the A-M frequencies are closest to the 72mHz range. They also produced a range "TV 5-6" and various others too, I think. The receivers must match the transmitter, and not all PR72b receivers are capable of operating in the same frequency range, even if you plug in the "right" crystal.
The PR72 receivers were made with a mono headphone jack until the end of their run, and even with the stereo jack installed near the end of their run, you are only hearing the same thing coming out of each ear of stereo headphones. The new jack was simply installed to allow the user to use stereo headphones without only hearing from one side.
Comtek is an AMAZING company, and will do whatever they can to fill you with knowledge of their older equipment, as well as helping you to modify it or keep it running. I have 12xPR72b, many of which have been "modified" with the stereo jack when sent in for service. They all work perfectly, and are more reliable and idiot proof than the PR75a (which have a tuning button - ugh!). I originally used a M72 with BNC, but now use the BST-50b base-station.
Please call them. They will tell you everyting you need to know and will help you become a customer for the long term.
Robert