Pic or splain plz cause on EB3s the thumbrest, if it has one, is completly on the pickguard (eyebrow shaped rosewood thing below the mudbucker) and never any spacer underneath that I have seen.
If you're talking about the metal bridge cover, then yes, some Gibsons, sometimes have plastic spacers. My Triumph that came with them (same bridge cover as an EB3), but I removed them because the added height made the case too snug in a worrisome way. From what I understand either most people got rid of these (even if they kept the bridge cover), or Gibson just didn't bother with them except for certain periods (my Triumph is a 75).
I also don't understand the bit about them making it level with the pickguard, the rest is completely on it and the bridge cover is nowhere near touching it. I always thought they were gaskets, like so as to not mar the finish (somewhat sharp metal edges).
It'd be easy enough to get some 1/8" plastic and trace a pair out.