If you can get hold of a thread gauge (looks like a keyring with a bunch of toothy keys/saws for checking how many threads per inch), then you should really be able to nail it.
That said, if it's ~5/8", there's a strong chance it's 5/8"-18UNF thread, which is semi-common in the gas business. Specifically, you might have some luck as that's the tank-side thread for a paintball tank, if I remember correctly. That said, you probably won't find a direct right-angle adapter, but you may be able to find a pair of adapters that goes from 5/8-18UNF (or whatever it is), to something more common, and back. Alternatively you could convert from that fitting (once you find what it is) to something more conventional and replace the regulator hose.
In either case, you'll want to focus your searches on the gas-handling businesses: Propane, CO2, welding gas, etc. - someone will make something that fits.