Yes, all the 78xxs are relatively scarce. Reason being, it was something of an interim reference. Heuer began using the 77x3 reference when Valjoux was supplying them with Vj 7730s, then fairly quickly moved onto the Valjoux 7734 with date. The reference schema in place at the time didn't allow a simple distinction between the Vj 7730 watch and the Vj 7734 watch (format was XXYZ, XX identifying movement, Y watch series and Z case material) but obviously didn't want two different watches to have an identical reference, so "fudged" it by using the 78 prefix to identify the watches with a 7734 in.

At much the same time, the Chronomatic movements were coming in and both Autavia and Carrera were undergoing a generation change (to the second gen in the Carrera's case and what I would call the 3rd gen Autavia, with 2446/3646 being the 1st and snap-back 2446/7763/7863 being the 2nd). Much of the effort, and production, went into those watches (the 1153 and 1163), at least as many as they could get movements for. Meanwhile, Valjoux were now supplying a "family" of 77xx movements and Heuer reflected this in the change to a 5-digit reference to show which movement is inside (e.g. 73353, 73453 and 73653 for the Carreras).

Even the Camaro, whilst it never received the Chronomatic movement, still changed its 7843 reference to 5-digit equivalents, as did the "economy" watches.

All this takes place over somewhere between 12 and 18 months, so the production run of any of the 78xx watches was short. And then, in the case of the Autavia and Carrera, they were playing second fiddle to automatic watches for part of that too, so as well as being short, the run was probably quite small. So we're never going to see very many. Interestingly, in all my years of collecting Carreras, I saw one watch, that I would call a 73453 marked as 7853 so it appears a handful of the later gen Carreras at least may also have received the interim marking - those may be the scarcest 78xx Heuers of them all.