That is not always a good sign
I'm in no way an expert for this reference, I hope you will get more competently feedback later.
Just two thoughts:
Here is a
bezel discussion and if you compare the original one with yours I'm pretty shure yours is not original
The crown isn't correct IMO but this alone shouldn't be a deal breaker
The same seller offers another SM300
EDIT: Picture replaced by a link to avoid potential misunderstandings
and claims the bezel as original.
IMO this is a service bezel which might be from Omega but is in no way original to the watch.
Either the seller doesn't know what he is talking about or he is claiming something which isn't true.
Both alternatives aren't acceptable
Buying a vintage SM300 is a tough job and it may take some time with reading and analysing until you'll find a good one
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