it says on the crown HEALTHWAYS L.A. CALIF. COMPRESSED AIR ONLY LCC3AA2250 L96855 PST 3662+
Sounds like it is most likely going to wind up being one of those tapered pipe threads. You will not find a new valve and would have to get lucky on ebay. No reason not to use the J-valve if it works.
Sounds like it might be a hard tank to get full. You will find very few LDS that will give you a complete fill for a pressure over 3500.