It is likely the redial work was done many, many years ago during a routine service. Very common back then when your grandfather was around.
It is also possible that you have the original rotor. Although the cal. 333 was a chronometer rated movement, not all examples have the engraving on the rotor. Most do, and for that reason the market favors those watches with engraved rotors.
In any case, Omega has no records to verify which rotor was originally installed on which watch.
One of the Ref. 2515 examples featured in your links does not have any numerals, like yours. I think that part is fine, it's just that the dial was refinished.
Hope this helps.
gatorcpa
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