I am going to assume this is a boutique technician and not a boutique watchmaker.
I just took this photo showing a sliding pinion on a stem that shows what I would call the "typical" fit between these two parts. This is not from a 9300, but given what you see here, do you think having one corner rounded off causing this is plausible?
I didn't think so...there is virtually no gap around the square portion of the stem.
I won't say it's 100% not the problem this technician described, but it seems very unlikely to me. There are a number of things that can cause this, from a missing or worn teeth on a winding pinion, sliding pinion, or crown wheel; deformation of the stem allowing the winding pinion to move away from the crown wheel under load (I have seen this with main plates that are heavily worn); the mainspring slipping prematurely and too much, etc.. Given the complexity of the winding system in this watch, it really needs to be sent in for service.
Cheers, Al
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