It's just us using shorthand and rounding up to the nearest thousand. It's very hard to be absolutely definitive for Heuer serials, so rounded up to 1000 is as good as the full 6 really - those of us with serial # fixations are still wading through quirks and complexities :) And I don't think anyone would claim to be always accurate to the 1000 even, let alone the sixth digit!

There's definitely some batching going on at that time though, we see that from the Chronomatics - Chronomatic Autavias typically start 141xxx, Carreras 147xxx and Monacos 157xxx. So how many watches were made between 147000 and 150135, how many of those were Carreras and how many 1153 Ns would be a pure guess. But probably somewhere south of the 3000-odd that simple maths gives us.

The hands on your example weren't used for very long either, which is another indicator of an early watch.

These are great watches anyway, comfortable to wear and easy to read - enjoy yours!