Ah, yes Mark, you are quite right, I forgot the Blue Montreal. I did have it in mind but got carried away with the 'sorting out of the black dials'.
So, the Blue 750.503B falls in the 'Convex barrel case with starburst S-steel' camp. I've only e ver seen this model with 'Montreal' on the dial. Which is strange, as it's convex barrel case brother, the black dial, has only ever been seen WITHOUT Montreal on the dial.

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.

Our very own Jasper of Classic Heuer has one for sale here: (unfortunately no caseback photo) But knowing Jasper, he wouldn't have listed it as an '-3' if it didn't have it on the case. But even he has fallen into the trap of calling it a '(Pre)-Montreal'. The MOAT notes it as a Pasadena!
http://www.chrono24.com/en/heuer/chrono-pre-montreal--id1630770.htm

I found these pics online:

Stewart

: Nice bit of work Stewart, it's always good to see someone doing
: some research on the Heuers, particularly lesser known ones like
: these.

: As far as the "non-" goes, I always read it as shorthand
: for, for example, non-Pasadena Pasadena, i.e. a Pasadena without
: "Pasadena" written on the dial. It just feels a bit
: redundant to write the model name twice so for ease of use it
: just becomes "non-Pasadena".

: The 750.503 Montreal was also available with a blue/blue dial too
: as the 750.503 B as well as the 750.503 N, so that's one to add
: to the list - it's missing from the OTD MOAT too.

: Not sure about the 1-3, do we have any pictures to go on? I'm just
: finding 501s and 503s in pricelists.