I think you are maybe correct to assume the 750 was added later, likely at the assay office. I believe this is the case (pardon the pun) previously discussed about mine.
I recently found this which has added a new twist to my search ...
http://info.hktdc.com/alert/eu0416a.htm
As a rule, jewellery items produced in or imported into France must be engraved with two hallmarks:
the producer's or responsible person's hallmark i.e., the specific hallmark of the producer or the importer in France which is meant to identify him; and
the so-called certifying hallmark which, until 1 July 2004, had to be engraved by either the State, an accredited body or the producer himself, certifying the percentage of gold, silver or platinum in items of jewellery.
I will elaborate at a future date but I’m inclined to think that different makers, importers, accredited bodies etc will have all had their own way of doing things during this period and sometimes there were anomalies
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