Well , generally the K-22 Target Masterpiece , later called the Model 17 does have a 6in tube.

And the K-22 Combat Masterpiece , aka , Model 18 usually does have a 4in bbl.

But we have seen 4in Model 17s and 6in Model 18s here in our members collections. Even factory nickel guns.

Here's my plain ol' 4in Model 18 with target stocks.