Have never seen a Model 21 choked(marked) IC/WS2?
WS1 and WS2 were developed by Winchester for SKEET shooting with the ammunition of the era, in all practicality, there is not much difference between WS1/WS2 and IC/MOD, at least none that will affect your hunting.
The question and answers are really a 'Tempest in a Teapot'.
If a "standard" SKEET Model 21 with either 26"(early gun) or later 28" barrels, they have very nice wood and finish were available with pistol or straight grip and only have a butt plate. Splinter guns if built for stock were older guns.
If you have actually found two Model 21's marked with the above choking, the primary concern would be for condition, condition, condition and stock fit. Either will shoot the same.-Dick
