This is something you see various trends in through the different 70s Heuers. As happens all too often, it's quite hard to find a unified theory for when, where and why it's used.

Where a watch existed before the Chronomatic launch in 1969 (i.e. the Autavia and Carrera), they definitely start out with what I would call the "classic" font, that we see on the majority of the 1960s watches:

The Monaco, without having those predecessors to inform its design, starts off with a variant of what I'll call the "modern" font for our purposes (inverted commas as for me it has echoes in the fonts used in the 30s and 40s, but as a label it will do in the absence of anything better).

All well and good so far.

Now, let's pick a watch that launches after 1969. Calculator in 1971, that will do nicely.

Modern font. Great, so our theory is holding together. And the Cal 15 Carreras, coming along just after the Calculator? Classic font, so that fits too. So the Cal 15 Autavias will be classic font too, right?

Ah. No. Well, that messes that part of the theory up :s

Oh well, at least we know that the new watches launching without a direct antecedent in the '60s will have the modern font, right Mr Montreal?

Oh. Poo. So the Carrera's the only one we can rely on to stick to our initial theor.... hmmm? What's that?

Humph. Right. So there's no consistency at all then.

And later on in the decade, we have a third design, used on Cortinas, Veronas, Jaramas, a couple of Carreras...

At least some of those have the decency to be the only type to be used on a model!

But for the two long-lived models, we have to take on board that all 3 types were used. All 3 Mark? But the last style was only used on some late Carrera 110.253s, right, not on the Autavia? Well...

Though it could be argued that only shares a layout with the version used on the Carrera et al, the typography is significantly different.