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.