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Thanks for the response.
There was an early version of the Competizione back when the OFMEGA badge first launched, ca. 1966, which may have come with a spindle resembling yours.
It subsequently evolved into a "bis" version.
There is an iteration of the Forgiato which fits your description. There is also the Gran Premio model which is above the Forgiato and below the Competizione, CX, Mistral; see catalogue linked to below.
IIRC at least
some Forgiato spindles are black oxide finish.
Found this image of a complete bottom bracket assembly with a ring/groove indicating the drive side like your example. There appear to be markings on the flat of the NDS spindle end.
The only online catalogue for these products I know of is here; it hails from 1983.
AFAIK close of play was ~1985.
http://velo-pages.com/main.php?g2_itemId=69479
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