The 21s pop ribs on occasion for good reason-the steel the tubes are made from feature a goodly amount of chromium, and that makes the job of soldering ribs difficult, as the steel is difficult to wet.
It is not a wives tale, and was mentioned in a letter John Olin sent to a customer pertaining to the forend loop coming loose on his gun. Are all 21s going to pop a rib? Doubtful. Do 21s suffer popped ribs as often or more than other gun designs from the same era? Without question.
Best,
Ted