: With regards to the 750.501-3...
: Like I say, it isn't in any OTD catalogues, but it is listed in the
: OTD MOAT.
: When I first saw pictures of them, I presumed they were Black PVD
: Pasadenas that had the PVD sanded/polished off. (This is still
: the case, you see many '750.501' marked Heuers that are
: stainless steel - these have had the PVD removed, probably
: because they looked tatty). But, I've seen sufficient
: 750.501-3's for sale that are in good/mint condition, that are,
: or appear to be factory stainless steel. Coupled with the info
: in the OTD MOAT, it's solid evidence.

With this paragraph Stewart, you've solved the riddle whether you know it or not!

It was Heuer's model code schema putting them into a bit of a fix. The terminal numeral of the numeric code usually denotes material, in these six digit codes as well as the four digit ones, with some exceptions like the divers. 3 is most common, and denotes steel, 5 is gold plate, 8 is solid gold and 1 denotes a PVD finish.

In calling the PVD 750.501 a Pasadena and the steel 750.503 a Montreal, a problem arose when they wanted to produce a steel Pasadena. Logically, that model would be a 750.503, but that number was already taken by the Montreal. Looks like the compromise was to append a -3 to denote a steel case to the existing 750.501 code for the Pasadena and differentiate it from the existing Montreal. Hence 750.501-3.

Makes complete sense to me now.