: I am not an expert, but the relief on the engraving looks
: reasonably deep and defined. The satin finish appears as though
: it is original.

The other caseback engraving would have been masked, so would remain well-defined as it was untouched. As Armando points out below, it is much deeper (I believe it is stamped) than the engraved serial number - which makes sense given that the stamped information is common to all the watches, so only the unique information is engraved.

Look at the area I've highlighted on the photo. See how it looks a little lighter than the rest of the case back? This is where the serial/limited edition number would have been found. And there is a fairly tell-tale vertical scratch below the 'e' of Heuer. I stand by my theory of this being a grey market watch originally, which had its identifying number removed.

It seems Heuer wised up to this practice later on with the re-issues - my Jack Heuer 1964 Carrera has the model number, limited edition number and a serial number, all in 3 different places on the caseback, making them considerably harder to obliterate than those on the earlier Monacos.