Sorry to not have any more info on the model, though you're doing a good job unearthing stuff without any help!

Yes, I couldn't find it in a '79 catalogue either, though the closely related Heuer 13-1 is in the one on OTD:

Given that the master table gives a price for it, I wonder whether this appeared in a price list rather than a catalogue as such. Looks like some more digging around could unearth something, but I doubt there is much information out there.

According to my source, the Vj 7734 that powers this watch according to the OTD table ceased production in 1978, the year before we're looking for it in Heuer catalogues. I guess from that it's possible that the 12 was a run-out model using some of the last of those movements. The production numbers would thus be governed by how many movements Heuer could obtain from Valjoux, who would also be supplying it to a number of other manufacturers - that could well have restricted the numbers of 12s that made it to market. It could also have limited the markets it was sold in.

That table could use some more work it seems - the Valjoux 7765 it states is in most of the other watches of the group was only introduced in 1983 so can't have been in those 1979 watches. It must be one of the predecessor movements; it would be interesting to see how they are marked, but I don't have one for reference.

That could also clear up some of the "made in France" assumptions. The watches at the top of the catalogue page above clearly state that on the dial:

Whereas your pics of your 12 state "Swiss made". Looks like the whole group have got lumped together and the made in France connection has maybe been made to all of them, but looks like it shouldn't have been. Heuer was in a fair degree of turmoil around this period, with a couple of changes in ownership and the records aren't any better than they were up to that point! So there's a fair amount of stabbing in the dark going on when investigating any of the less well-known watches from this period. We know for sure that Heuer sub-contracted work to at least Monnin in France for some of the dive watches in this period, and possibly other companies there for other chronographs. It would also seem some watches were produced in Switzerland too, with the 12 being one of them - I think my theory of a deal with Valjoux for a job lot of 7734s is a pretty credible one, but someone may come up with a better!

So, sorry not to be able to provide any concrete information, but at least there are some ideas and questions in the above and that sometimes helps too! Have fun continuing to investigate.