I don't know anything more than anyone else here, I am looking at the same photos as everyone else. This thread, and the fact that I picked up a MGC recently which I am leaning towards converting into a Hoth, got me looking at the pictures again.
I originally thought I saw some glints from the screws, but I am rethinking that. Why? Because I don't see the reasoning behind leaving a plate with screws exposed like that. It's pretty sloppy, in my opinion. I now believe that a knob from a turn table or some such thing was used, and it covered the whole plate. I think the glue circle came from the center post of a knob like this one (I am not saying they used this exact one!):
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I think they installed the disk, painted it with the screws, and installed the knob. You can see a spot of black within the glue circle. Maybe when the knob fell off it pulled up that bit of glue, exposing the back paint. The other thing though, is when I look at the photo of Luke holding that blaster in the snow, I kind of see a lower step like you would see it they used a knob similar to this one:
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I really like what Chubs did, I am going to follow his lead with one of these knobs. At least until a better photo comes along. It would be an easy fix right?