One of my 3" 21s is a pre war "Joe Knapp Club" gun built for Michigan mega gunner Lester Ruwe. It is so petite, I have no idea how the stock has survived almost 80 years of use. It feels like a 20 gauge in your hands. Joe Knapp was a founder of Ducks Unlimited and had a duck club in North Carolina where his ultra rich cronies were conferred membership in the "Joe Knapp Club". Their guns were adorned with gold escutcheons with a bust of Joe Knapp sculpted into the gold.

Morris Hallowell has a decent Duck for sale. Priced well!

If I were in the market for a 3" Model 21, Hallowell's gun is the one I would buy.

Galazan generally has one or two, but he's pretty proud if them

21 duck
 

I have a model 21 Duck with 32" barrels and a straight grip made in 1951. I have to say its a great shooting gun. I shoot 2 3/4 ammo in it and have fired 3 inch loads but think the shorter stuff works as well and is a little softer on my shoulder. I was not looking for one and happened to be in a shop that specializes in Black rifles, The owner just got it from a young lad Who was left the gun by his Grandfather. The shop owner was getting his hacksaw out and was going to cut the barrels off and make a coach gun. The only problem I have ever had is I let the forend iron get loose and the wood developed a crack at the little tits near the forend. Its in the shop now being repaired. I use it as a trap gun and for Pheasant hunting. Never took it waterfowling yet.

Sounds like a great gun