I think they look like big blind rivets similar to the ones I posted earlier in this Thread. Big donut shaped top, big hole in the middle, but the only photos we can find don't really show clearly enough to make a definitive call on what they are.
These were painted silver
The idea was that the metal was too reflective and screwed with the camera trick of the blade's reflective material. This was also the case with the severed hand saber, though that may have been to cover up missing plating.
This ESB stunt doesn't have any visible hex-head allen bolts, I've wondered if the rivets are holding the blade assembly in the body or just holding the tubes and clamp together.