Thanks for everyone who helped out... I believe this is the long and short of it...

7750 Movement series model no's and description. Listed in no particular order.

1) 750.501 Pasadena (later models didn't have Pasadena on the dial)
Black dial.
Concave lip barrel case, Black PVD.
(In OTD catalogues 1979, 1981 Speciality, 1982 Speciality)

2) 750.501-3 Pasadena (only ever seen with no name on dial)
Black dial.
Concave lip barrel case, circular brush effect Stainless Steel.
This model causes confusion, as it clearly has the model numbers of a Black Pasadena with '-3' added, but is Stainless Steel. Heuer added '-3' to denote a stainless steel. Because of this confusion, it is sometimes referred to as a Montreal.
As Mark Moss explained...the Heuer end number usually denotes the finish, '3' being stainless steel, '1' being Black PVD. etc. (not a hard and fast rule, but used extensively by Heuer)
The Pasadena was originally only available in Black, hence 750.501, but when they wanted to make a stainless model, they had to add the '-3', as the number 750.503 was already taken by the Montreal. Quite why Heuer wanted to make a stainless version is unknown, as there already was a stainless version, under the 'Montreal' name, and as they had dropped the name from the dial, it is a little perplexing. As always, Heuer was in a bit of turmoil around this time, and many anomalies occurred.
Not seen in any Catalogues on OTD at time of writing.

3) 750.503N Montreal (later models didn't have Montreal on the dial)
Black dial.
Concave lip barrel case, circular brush effect Stainless Steel, Montreal.
(In OTD catalogues 1982 Speciality)

4) 750.503N Montreal (yes, same ref. no. as above but different case) (only ever seen with no name on dial)
Black dial.
Convex barrel case, Starburst Effect Stainless Steel.
(OTD catalogue 1977, 1978, 1981 Speciality)

5) 750.503B Montreal.
Blue dial.
Convex barrel case, Starburst Effect Stainless Steel. Only ever seen with name on the dial and in day/date form. Signed Crown. A rare beast.
Not seen in any OTD catalogues (That I can find!)
http://www.onthedash.com/Guide/_Chronographs/72.Montreal/p.Ref_750.503B_Blue/?show=23Mont750503B.jpg

Notes of interest

Named & no-Named Differences
It has been noted that the dials between the 'named' and 'no-named' have small differences.
The 'named' dials have small rectangular shaped markers at the 12, 3, 6 & 9 hour positions. The 'no-named' models have dots.
The 'named' dials have longer seconds markers between the hour markers on the main outer dial.
An example has been found that 'breaks' the above observations, it is understood that this is most probably a transitional model, and further anomalies may be found in such models.

Day & Day/Date
Most models are regularly seen and documented to be available in both 'date only' and 'day/date' guise. The scarcer models (ie. Blue dialed) have only ever been seen in day/date. This doesn't mean that it doesn't exist, just undocumented at the time of writing.

Crowns
This hasn't been extensively researched, but it seems that all models had unsigned crowns, with the exception of the 750.503B Blue Dialed model. So few of these are documented it is difficult to verify that this is a hard and fast rule.
Always remember that some may have been replaced at Heuer service with a signed crown.

Caseback markings
Some models had '280 SL' marked on the caseback along with the '750.XXX' ref. no. Some just have '280 SL'. Due to lack of material, this cannot be fully investigated.

Other
It is also noted that in the 1982 Speciality Catalogue price list, that the Montreal 750.503N ($475) and Pasadena 750.501 ($485) models were the most expensive models in the catalogue. More expensive that the Autavia 11630, Autavia 11630 Diver, Calculator 110.633 and Skipper 15640.

Also be aware that some Black PVD Pasadenas end up being de-PVD'd to tidy them up, making them appear as no named Montreals. An easy mistake if it is a no-named version.

Stewart
PS. Apologies in advance if I've made a glaring mistake!
Thanks to Jeff, Mark, Tim and anyone else who helped out.
Also, please use this as a guide, & always do your own bit of research before any purchase, etc etc.