Early Model 1100 and 870 20 Ga. shotguns were built on the 12 gauge. frame. The serial number on these shotguns end in an "X" or a "N". The barrel guide ring (gas cylinder) brazed to the barrel, was a 12 Ga. ring. These barrels are no longer available from the factory as they were discontinued in 1978. The barrels sold now are for the LT 20, which is built on a narrower frame. Its smaller barrel guide ring will not fit over the large frame magazine tube.
On our site we have a listing of parts dealers that may offer replacement barrels for your older 20 gauge under "Specialty Parts Dealers".