There are two types of barrels for small frame 20 gauge 870's.
Standard and Special Field.
Special Fields are no longer made, but their barrels can be found.
Mag tubes are shorter on them so the bbl ring that the magazine cap screws down to, is farther back.
Special Field barrels only fit special fields (in 870).
1100 Special Field bbls fit standard and Special Field 1100's (within gauge and frame- standard and LT)........since the forend holds them, mag cap screws down on end of forend. 1100 special fields also have a shorter mag tube. IIRC 1100/870's were shortened to hold one less round.
The old 21" fixed choke 20 gauge 870 Special Field.......was pretty sweet.
FWIW used 870 Express Remchoke bbls sit in gunshops, used like new, at $150 sticker priced. Two 26" sit at my LGS.