The guy realizes that CO2 comes through really cold. What he doesn't realize is how little we use. Tell him thanks for the advice, and then have him remove the nipple and sell you the CGA320 nipple and nut.
It shouldn't have left hand threading. It might have 1/2-27 threads, like other older Harris Group models. I have one or two 3104's in the workroom - I'll check a bit later. Western Enterprises part #'s CO-3 and CO-4 (2" and 2 1/2" CGA320 nipples) are standard 1/4" NPT thread. CO-7 is the 1/2-27 thread. They all use the same nut - CO-2. And they're available through Airgas.