Phoenix Tech's web site indicates the Kicklite recoil reducing stock will only fit the 870 Remi 12 gauge and not the 20. Anybody tried to instal it on a 20? I'm not sure why it would not fit.
The 12 and 20 used to be on the full size receiver, but that changed in early 70's.
Since then 20 gauge 870's have been on a smaller receiver, called the LW.
Yes, older small 1100's were LW's too (early 70's). They changed to LT in '77.