The 21 slide is a little wider. They beveled the bottom edges so that it could fit on the 22 frame. When they first announced the 37 it was supposed to be equal to the 17/22/31 gun all the way. So now they have kind of a mut which means alot of holsters won't work
Guess I'll have to wait and see if they do come out with the 38 in 19 size... that and see what the holster makers do.