I don't know how tight Lee's tolerances are and if all # 19 lee shell holders will work with 6.8 brass.
I don't have any empty brass other than SAA to try.
It seems like there are two different dimensions that these shell holders have to take into account.
They have to actually fit the rim. That's width and the thickness of the rim.
But the other thing is height. If the height isn't right, then primer can't be seated fully.
For instance, I started out trying to use the #1 holder. It seemed to hold the brass by the rim just right. No wobble or popping out. So I seated a primer. Looked OK. Then for some reason I thought to try the hard flat surface test. You put the primed case down on the surface and then try to wobble it. If it is seated flush or below flush, it will be solid on the surface. If the primer is protruding even a bit, it will rock. The one I seated using the #1 holder rocked. The #1 must hold the case just a little too high.
I honestly don't know if that could be an issue with the #19 but I would examine it anyway. With the #1, I was inserting the primer with a full squeeze and it still wasn't enough. You would like to have the primer get firmly seated before you run out of seating pressure. That's kind of the point of hand priming. You want to be able to easily feel what is happening and get it "just right." Not just jam each one in there to the full limit of the tool!
I'm not saying you guys are wrong. In all probability you are right. If I can't come up with a #21, I'll probably try a #19 myself!