Ok there are two types of tank fittings you are likely to find.
The older fitting is a Prest-O-Lite, POL for short, some folks call it a Put On Left because the thread is left handed (Righty Loosey Lefty tighty)
The newer fitting with outside threads is clled an ACME by the way and includes several safety features lacking on the older POL fittings.
YOU MAY, and I stress the word MAY have problems getting a POL tank refilled.. For one, it will most likely need re-certification if it has not been re certified, I think ACME has been out longer than the recert period for new tanks. Second... A lot of pump jockies may not know where the POL filler is any more having never seen one before. (This says a lot about my opinion of pump jockies and very little about your tanks).
Of course modern ACME tanks will accept POL hoses so if you can have troubles you can always trade 'em in on new tanks (or properly dispose of and buy new tanks).. Just so you know.