http://www.chronocentric.com/forums/heuer/index.cgi?md=read;id=53201

: Hi Michael.

: 249xxx is by no means a late 1533; in fact, it becomes the earliest
: Cal 15 Monaco I have on my list now, the previous "record
: holder" being a 250xxx serial. So that's entirely
: consistent with early 1533s being in cases actually marked 1133.

: The other Cal 15 models are in similar sort of ranges:

: Carrera - 253xxx
: Autavia - 243xxx
: Calculator - 266xxx
: etc

: I agree with your assessment of the dial though. I'd go as far as
: to wonder whether, with its blue register, the dial was
: originally branded something other than Heuer and has been
: "converted" to the main brand to gain a higher sale.
: Though the caseback does have the Tool 033 markings, which the
: off-brand watches don't usually have.

: All of which brings us to your watch. And I'm afraid that that
: seems to have been a recase - a 168xxx serial is definitely no
: later than 1970, at which point the Cal 15 was still the best
: part of two years off. To the best of my knowledge, they hadn't
: even conceived the movement by then.