It's probably because the connector port threads of the old(er) tanks are different than the newer style, propane companies have gone to. I remember needing to exchange older 20# BBQ tanks for newer style, but don't exactly remember when, but '95 time frame could be correct with what you found when looking for gauge.
I was thinking about a propane gauge just yesterday and need to look at my '84 Prowler 30# tanks to find the Manufacturer Date/Year that should be embossed on them at the top handhold ring. It is used and don't know if PO did any tank/regulator swaps when he did restoration on it, to sell.
Del & Jerre w/Isis our English Bull Terrier
The Cougar's Den
1984 Fleetwood Prowler "E"