I'm fortunate in that my eyes are as good as they ever were, but I still sometimes have to resort to a big loupe to read some of the serials off - and wander round the house and garden trying to find where the light is just right to read it! On the watches that time has been kind to, it's easy to see (my 1158, for instance, is as clear as day) but it seems remarkably easy to obscure the number with a few decades of handling and wear - not too surprising after all, I suppose.

Cal 15s have a definite date horizon, at least for their introduction. They were first sold in 1972. The end date is a bit trickier - they seem to fizzle out in the late 70s after having their swansong with the Monzas and were outlived by their older brother the Cal 12. The last time I have seen them catalogued is in 1977, in Monzas and the Skipper.

With a 250xxx serial, though, it's more the start date than the end date we are concerned with. It looks like numbers in that sort of range are from the year of introduction (i.e. 1972). 1972 also looks to be the year where the numbering system begins to settle down and use all the available numbers for watches - in earlier years, we see largish jumps in serial numbers year on year, but the increase is smaller from circa 1972 on. My gut feeling is that the early 70s were good years for Heuer, with series watches in full swing and selling well, so I don't think the sales were worse than in the mid- to late-60s - that's why I presume that all available numbers were being used from 1972, but allocated blocks and skipped numbers before that.

As I write, though, it does occur to me that I don't know much about the stopwatches and how (and indeed if) those were numbered. We know from the forum members who went to the museum visit that Jack Heuer said they were producing 10-20 stopwatches for every watch throughout periods of their history, so it's possible that there was a shared numbering scheme pre-1972 and they diverged at that point - that would also explain why we see smaller increases in serials year on year later on. All conjecture right now - does anyone have a 60s/70s Heuer stopwatch that they could check for a serial?