Which I find interesting. I have actually drilled these holes in a replica lower, and I drilled into the bottom plate because it was too close to it!
Which gives me more reason to doubt the bracket explanation.
My theory: (bare with me, it's a lot of speculation)
Roger Christian has told the story of how he built the first prop. I know he isn't the most reliable, but some of the details must be correct. He said that he found several in that box, and slapped the grips and bubble strip on the first one. But I wonder how many of those several were actually 3 cell flashes and how many were only 2 cells.
He then said he was instructed to add the D-ring; which means the grips were already on. I think this debunks the idea of why they just didn't put the grips over the holes. My thought (as has been discussed) is that they tried to attach the D-ring to the side. They may have even tried drilling holes and bending in a D-ring directly into the holes ala the MPP and Mk1. When that didn't work, they plugged the holes.
Now, why two rivets for each hole? I'd think larger rivets would go to far under the grips, and push them up (which, I imagine they removed one to attach the ring). Two rivets per each hole means a longer plug that wasn't as wide. These holes could also be from an attempt to add a spinning blade as suggested.
Now, for the next problem: Why use *that* one for the Hero? Well, Roger Christian said that he sent two to Tunisa. I wonder if there were *only* two 3 cells. He put both of them in the box to Tunisa, and they just grabbed the one with the rivets on set. The second ended up becoming the Elstree saber when they were filming back in London, and they realized they didn't need two Heros.
This is all speculation, but it fits all the facts that *I* know, and it makes sense to me.