Just looked at my YashicaMat and 635.
The 635 has what looks like a soft black thread running around the inside of the back cover (kind of flattened into the groove). I've seen this before on older cameras.
The Mat has something similar, but I can't quite make it out. It looks like a very fine lining of velvet-like material.
In any case, they both have a U-shaped channel running around the outside of the back cover, so I doubt you'd get leaks anyway. As Francois says, a well-designed back will stop the leaks, and these are well-designed.
If you did happen to get leaks, it's an easy fix.