The used Wingmasters are probably 2 3/4 or 3" chambers. They are better guns IMHO than the 870s being made now.
Why 3.5"? 3" is adequate for any hunting you want to do. 3.5" is useless in my estimation. Just another sales gimmick. 3.5" shells pattern poorly from what I have heard.
Besides the 3.5" gun has some extra parts that can cause problems. The bolt is modified to handle the extra length shells.
And 3.5" shells just hafta hurt