I'm not familiar with those particular tubes you mention, but beam power tube plates are often made of two different pieces that are attached to each other by bending tabs in piece "A" through slots in piece "B". I see this in several 807's and 6146's I have on my desk at the moment (they're usually RF tubes, not audio, but they are beam power tubes.)
These holes usually aren't in the parts getting the beam but are external. I mean, you cannot see the cathode through the holes, you just see out the glass on the other side.
If you look at the holes (look at both sides!) do you see any such attaching-two-parts tabs in your tubes?
Tim.