Whilst a number of watches prevented me from getting a unified theory on the 6 digit model references, it was only the Autavias I remember throwing up problems when trying to resolve the 4 and 5 digit ones....

Heuer had managed to divorce the scale from the model reference on the second generation Carrera; e.g. what was the 3647 T became simply an 1153 S. The Tachy scale became standard so was already implied in the nomenclature, no need to specify it. Non-standard scales were available on request, notably the Deci scale. Potentially the combo scale we saw recently on an 1158 was a request for a Pulso version - the original Carrera Pulso seemed only to have been available to special order, certainly at the end of its life. So the key to the model reference became dial colour for the Carrera, and most of the subsequent models followed suit (Monaco, Silverstone etc).

Not so the Autavia. The Autavia was defined amongst the Heuer chronos of the time by its external bezel, and further by the scale on that bezel. That was probably a contributing factor to the lack of variety in colour of the early Autavia dials, trying to keep the complexity of the alpha suffix in check! And later the lack of "portability" between dials and scales - if you wanted a Tachy scale, you got a Siffert dial, MH on the other hand would net you black. This carries on into the Cal 15 models too.

It helped Heuer keep things relatively simple for things like ordering and assembly, but didn't necessarily give the customer all that much choice and flexibility. So I could totally understand them offering the ability to order different bezels (and charging for the privilege no doubt) but I don't know if this is something that they actually did, and whether a decompression bezel would have been one of those options. Equally, there is a possibility that this is a later service bezel, which might explain some differences between it and some of the pictured original versions, e.g. the lume dot at 12.

Interesting that the watch pictured in the catalogue has the same slightly off-looking, unstylised "Heuer" text in the shield that we see on the watches in the metal. I wonder, what with that and the unusual (for an Autavia) lume markers, the P dials were supplied by someone other than Singer or whoever the usual Autavia dial supplier was? I don't think these are rated any deeper than the other Autavias, so don't think this would be a technical reason re sealing? Darren's watch on the other hand has a more familiar Heuer script.

Price lists are always a good place to check for this sort of thing. A couple I have from '73 only show the 11630 (in M/H, T, P and GMT guises), as does 1974. 1976 does show the 1163 as well as the 11630 but unfortunately does not specify the bezel. Interestingly, it is marked "Whilst supplies last" (actually, "Zolang de voorraad strekt" but I translate for your convenience!). Does anyone have one from 1972? If the 1163 P is in there, it's normally a pretty solid indicator that the watch actually existed.