I'm always a bit wary of trying to be definitive on these late 70s/early 80s watches. Heuer seemed to be careering from one crisis to another (to be fair, so was most of the rest of the Swiss watch industry) and models were introduced and deleted without seemingly much rhyme or reason.

The case, hands, markers and tachy ring look to be the same as the 13-1 on this page of the 1979 catalogue:

That has an unusually large Heuer signature too. And we've seen unsigned movements on these chronos as well. Which brings us onto the subject of the movement - these normally have a Valjoux 7765, whereas the pictured watch uses a 7734. Heuer did use 7734s though, so that's not enough reason to rule it out entirely as being genuine. That is the cheapest possible version of the 7734 too (Valjoux sold them to the watch companies with varying degrees of finish and refinement, reflected in the cost - in fact, movement manufacturers often still do), which could be one reason for not putting their (Heuer's) name to it... It does nag a little that we only ever see these late run economy watches in catalogues with 7765s, but we also know that some Heuers never made it into the catalogues. That is the cheapest possible version of the 7734 too

So what am I concluding? Not sure I know myself! Some of the bits look like genuine Heuer parts, but I don't have enough info to say for sure about the whole watch. One thing is for sure, this watch and the one on eBay aren't any sort of Autavia!

I'm going to sit on the fence a bit on this one :) Some of the other forum participants have better knowledge than I do of this transition from Heuer to TAG-Heuer, so maybe one of them will be able to be definitive.