I don't want to become embroiled in an argument, but if I may make some observations over this period in Heuer's history.

For the majority of the 70s, Heuer made it nice and easy for us, with most of its watches having the model reference (e.g. 1153) engraved between the bottom lugs and the serial number between the top lugs. They started this practice at Heuer in the late 60s; previously only the serial number was engraved.

All well and good, and most of us look for this when we are considering a watch, as has already been noted here.

However! (I'll use a Carrera example, as ever, because those are what I have in my collection). Roll on 1977/8 and Heuer is replacing the 1153s with the 110.253s. In most respects, this is more or less the same watch, but as far as the case goes, there is a significant change - the model and serial references are no longer engraved on the case, but on the caseback instead.

We see some other watches adopting this practice around the same time (or a bit earlier for some models), such as the Monza. It would be tempting to assume that this was because the PVD process would obliterate the numbers, but the same thing is happening with the steel 110.253.

At the same time, Heuer is starting to produce quartz Carreras in more or less the same steel case, only shallower. Except these still have the model and serial references engraved on the case. To further confuse the issue, though, the quartz watches have an unsigned crown. Bang goes another thing we look for as a sign of authenticity.

So what is pretty much a truism re signed Heuer cases from that point on becomes dependent upon the model. The same goes for crowns.

The end result is that our authenticity checklist suddenly gets a whole lot longer (and more complex) after the mid-70s.

This all goes to set the scene, showing that there is a degree of flux in Heuer signatures by the time the 5100 watches are being introduced.

Picking Carreras, again, I expect a 510.511 or 510.523 to have an unsigned crown, an unsigned case and a caseback with an engraved model reference number. There's a possibility some had signed crowns from factory, but the majority would appear to be unsigned.

Here's a pic of mine, so this isn't just a wall of text! :

Those are the hands I expect to see on a Heuer with the 5100 movement too. Again, I stress the word "expect" - it doesn't preclude the possibility of other hands coming out of the factory, but these default Lemania-supplied hands are pretty overwhelmingly what we do see on Heuer 5100-powered watches.

Bullitt, I do wonder about that serial number. Heuer serials tipped over into the 400xxxs in circa '79, having taken ten years to get there from ~ 110xxx. I'm not sure when we are pitching production of 5100 Silverstones but I guess we're looking around 1983, like the other Heuer 5100s? Four years does seem a very short time to have gone to serials with 800xxx but it's quite possible Heuer rejigged their numbering scheme in that period, hand in hand with changes in ownership. I don't have many other serial numbers from this period for comparison - maybe someone else does and can confirm what range they are in for comparison?

So, in conclusion, I'm not drawing a conclusion at all. What I hope I'm doing is pointing out that all our hard and fast rules from the early 70s aren't really rules any more by the time we get to these. It all comes down to a familiarity with the specific model and hopefully a large enough sample size to see genetic traits coming through. I certainly don't have the former with these 5100 Silverstones and I wonder whether we have enough of the latter too.