Many many thanks! Going to send it to my watchmaker first. Btw it is 36mm wide.

: Hi Tom.

: It's an interesting one, but with the catalogues we have I'm not
: sure we are able to pin down the exact model for you.

: The dial is very reminiscent of this 5408 from 1945:

:
:

: The case looks similar too, but that 5408 has a 10.5 ligne
: movement, which is less than 24mm - given how the movement fits
: in your watch, I'd say we're not talking the same model, but a
: larger version thereof.

: We also see from catalogues of this period that there are plated
: cases with steel casebacks, like this 342:

:
:

: The dial has the wider version of the Heuer shield, so I'd agree
: that it's not one of the very early ones - somewhere from the
: late 30s to the mid 40s would be about right, I'd say. There
: seems to be at least two series of serial numbers in parallel at
: the time - if it's one of the 4xxxxx ones, low 450s and earlier
: are likely to be pre-war or during the war, higher 451s the
: post-war period. I don't have a good handle on these older
: serials though, so that's about as rough a guideline as you will
: get! The 7xxxxx serials I'm even less clear on...