Monnin cases were used by well over 100 different brands, the dial on this one is reminiscent of the O&W version, although the hands are Heuer style. (But don’t look quite right) Aurore and several other brands also had an almost identical dialled watch.
Is the bezel insert a Seiko insert, I think it might be.
I would say franken watch rather than prototype,
As with all things Heuer and Monnin I would say Blackie here is you man in the know.
Edit so far 115 brands using the Monnin case have been noted https://www.mwrforum.net/forums/show...onnin-Examples