Thank you
@oddboy for your question.
I don't have a definite answer, but while I own some of these watches, my own database tells me that the early 2998's came with a 17,301.xxx/17,302.xxx serial number and my 20 years of observation tell me that the 17,76x.xxx movements were service movements. Back in the early 60's, they used to swap the movements instead of doing the work, I believe it's as easy as that.
It is sad, but most likely true. In those days, they didn't give a damn on originality there in Bienne.
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