Why numeric marking on Tissot Sideral?

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Is this common for a Tissot Sideral?



Or was it marked by the prior owner (my grandfather)? I’d ask him but, well….
 
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It might be a watchmakers service date eg May 1st 1974? Looks hand engraved.
 
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Not factory and I don’t think a watxh maker would mark externally.

although that is not a metal case so maybe it is a watrchmakers mark.