I suspect this comes down to volumes as much as anything - we have heard anecdotally from Jack Heuer that the Monaco at the time was not a runaway success, whereas the Carrera and Autavia had longer histories for Heuer to estimate sales against and were selling strongly.

There are other occasions (hands, markers) where Heuer seems to "use up" existing stock of components even when they have a new or redesigned model, and it looks to me like the same thing is happening here with the Monaco cases. The situation isn't quite the same, as the Cal 15 models were intended to complement, not replace, the Cal 12s. What would be interesting would be to compare serial numbers to see whether they used 1133 cases at the start before 1533 ones were available, or once the initial batch(es) of Cal 15 cases were used up.