Advice needed on how to proceed with a Cal 321 2915-1

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No problem at all. That clutch can be replaced easily. Tissot/ Omega . not that hard to find.
Oh I wasn't suggesting that it was a problem, just an indication that movement parts have been changed as well as the dial, hands etc. It is an interesting and valuable watch, just not one that is easily put back to the condition a collector might aspire to own.
 
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If this is to keep as a family heirloom I would do the following:

1. Full movement service.
2. Full clean of the case (without any polishing).
3. Leave the watch without the bezel. The missing bezel is part of the watches history and possibly how it was worn. I’d continue that trend.
There is a large element of personal preference here and if you prefer to install a suitable bezel, you could well go with @Spacefruit’s suggestion as this will be fairly cheap and is fully reversible.
4. Full restoration of the bracelet (if it is to be used) or store the bracelet away and put it on a leather strap.

The above will minimise service costs and also minimise further movement away from the watches original condition.