Which part of the top are you talking about?
Down the sight path is a tube with TomTit pistons at each end, with the original Ruger sight in the rear.
Covering the ejection port on the right side is the TomTit pontoon piece with signal cable reduction adapters in the center.
And, of course, the left side is what we're trying to figure out.
I haven't seen much talk about the piece on the back of the blaster, though I may have missed it. My working guess right now is the rubber foot from the Michelle record player that other parts have come from. I am in the process of ordering some Bright Star IsoNode feet right now, to see how they look and work. It could just as likely be a bolt cap or any number of model pieces.
Also, my seamstress is sending me some rivet caps that she thinks might work. So, I'll share what I find. Though it won't confirm anything for actual use.