Joe, I don't know what they would fit either. I just know that the National machines I have....a Reversew from 1937 & a Montgomery Ward from 1949 use a flat shank 21. A year or so ago, there were round shank 21s for sale on ebay & I just figured they were in the wrong case. But, if you've got 3 cases marked 21 & they're round shank, I'm guessing they couldn't ALL be in the wrong cases! If it IS an age thing, it sure would be less confusing if Boye would have given the flat shank ones a completely different #, huh?