Caseback year changes

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I recently received a nice surprise from Omega (via some very kind assistance from a UK member!) in that my movement number 20,522,XXX was confirmed as a reference 105-012 with a production date of Sept 1964.

I purchased the watch from my father's friend (original owner who bought it in Africa). Does anyone have insight on when Omega switched the date stamping from -63 to -64, was it at the end of the calendar year, or whenever they used them all up?

Thanks all!
Edited:
 
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What is the Watch ref.?
For example, on a Speedmaster ST 105.003, the inside of the back is ST 105.003-65, even though the archive production date is 1967.
If you can provide a little more information, you will get an answer from a knowledgeable member.
 
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Mega whoops!! Totally missed that essential bit of info.... it's a 105.012!

Thanks for spotting that omission!
 
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A serial number would sure help. Maybe edit the original post trying to provide comprehensive information and some photos.