Ive read what's there a few times, and can read it different ways, so please bare with....
The "tubes". Look close at the diagram above Jim posted. Is it the brush cover plate, or the armature bearing plate? Because the brush plate does have tubes, but they don't secure it.
The armature bearing plate on the other hand; that I could see being riveted on. It would seldom get removed. But check the back side of the "tube" for a screw head, or a staking/rolling of the end. I say look for a screw head from the back because it may also be a low profile head on a threaded insert. They are used in tight situations instead of a normal nut or the screw head on the outside because it's flatter than a screw head or nut.
Let us see it to be sure though, but if it does turn out to be a rivet, drilling the head off is standard procedure. The PITA is if it spins in place.
After? Could the tube be threaded; tapped maybe?
