I don't have any specific knowledge of UG case construction techniques; I'm just trying to reason this puzzle through.
I imagine it would be difficult to reliably bond solid gold lugs onto a stainless steel case, given that the lugs are subject to the most stress. So I'd start with the assumption that the lugs and case are the same material (stainless) for a strong connection and that the lugs are covered with gold for some bling.
This picture (from above) looks like the lugs were first gold-filled and then inserted through a hole in the case. The lugs would need to be bonded on the inside (Welded? Press-fit?) If you're comfortable removing the movement from your watch, you might be able to see evidence of this connection.
View attachment 844174
As far as the bezel, other manufacturers of that era often applied a solid gold bezel to a stainless case, so I would think UG did the same. My theory is - solid gold bezel, gold-filled lugs.
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