Hi Mich, first of all congratulations, that case is such a beauty - so clean and perfect lines, not a single hairline to be seen anywhere. Not a Monaco fan here, but although I would not buy or wear it personally, that deserves its share of accolade.
I agree, all cal.11 etc. cases that we've seen over the years have much "rawer" lettering for serial and model numbers. This one almost seems laser etched instead. And no signs of wear from the bracelet at all.
Both your suggestions sound realistic to me, and maybe you could query the TH vintage dept. to confirm the second? If it is such a late work, they may have some records of it, especially if they've gone as far as re-etching the original SN. I have read Omega does this, i.e. when they change a case they write the original SN on the new one so that the movement-case pairing still matches - maybe it's a standard industry practice?
Ein schoenes Wochenende,
Fabrizio