Here's a few....
Some shooters/collectors feel the 21 was the best of the American sxs guns. I don't believe you can wear one out simply shooting it.
Winchester also did some nice custom work on 21's....12 gauge 2 barrel set no. 6 engraved with B checkering.
"Winchester's Finest The Model 21" by Ned Schwing is the definitive book on this shotgun and well worth reading by anyone interested in 21's.