Workers must have reached into parts bins and grabbed what they needed to build a gun. My 1953 vintage 12-gauge Skeet Gun has that early-style forearm release roller. My 1941 vintage 16-gauge Skeet Gun has the later style.

Model 21 serial numbers are all over the place but that is very late for the early style release. If you want 21 info the best source I know of is the book on 21's by Ned Schwing even though it's 30 years old.

I actually bought the Ned Schwing Model 21 book back during the 34 year gap I didn't own one. About $25 on the used book rack at my local pawn shop. Probably a better buy than any of the guns!!

Used hard cover copies of the book are about $250 on ebay

Cylinder and modified is actually a fairly common combination for prewar double trigger 26" 21s. Prewar Model 24s were also commonly bored cylinder and modified in 26" models. The Model 24 26" guns were changed to improved cylinder and modified after the war.

Dave it just became "Special Order" or possibly "Custom". All kidding aside I like them over the typical roll style. No real solid reason really.

Up through the November 1932 Winchester catalog the chokes offered were -- full choke, modified choke and cylinder bore. The 1933 Winchester catalogs added improved cylinder, improved modified, WS-1 and WS-2. The Model 21 Skeet Gun first appears in the 1933 catalog in Tournament Grade, Trap Grade and Custom Built Grade. I have a Winchester Skeet brochure that discusses 1932 wins with the Model 21 Skeet Gun and describes it bored improved cylinder in both barrels?!?

My father has a 16 marked cyl/IC.

I had a 12 gauge skeet gun bored the same way. It is a better choice of chokes than WS1 and WS2 because the WS1 muzzle is sometimes paper thin and can be easily damaged. The constrictions of cylinder and improved cylinder are very near the skeet chokes.

Winchester chokes tend to be more open than most.