Hi and welcome to the forum. And congratulations for doing it the right way round by raising queries before buying, it's rarer than you might think!

I'll start off by being definitive. It's not a 1966 Heuer. Heuer weren't making any watches like that at that point.

And then onto the other concerns, which aren't quite as black and white.

- "Antimagnetic" and "17 Jewels" on the dial. This is a concern as this the number of jewels is something Heuer only very rarely put on the dial. I don't remember them putting "Antimagnetic" on at all. Lots of other manufacturers, often less well-known, did however.

- No "Swiss", "Swiss Made", "Made in Switzerland" or such on the dial. We'd expect to see one of those on a Heuer made in the 50s or after. So unless we're pitching this for the 40s or earlier, then that's a concern.

- The crown is a later addition. It's anachronistic with the bar pushers - Heuer crowns of that period were unsigned.

- We do see Arabic numerals at 6 and 12 on some Heuers, along with similar stick markers to these but typically on 50s/early 60s watches.

- Which leads us onto the scales on this watch, which sort of predate the period Heuer was using the above markers. Some did combine Tachy scales and 5 second numerals as here, but I've never seen the red 1-20 scale. Not even sure what that would be measuring. More like the scales we see in the 40s, maybe even the 30s.

- Heuer's serial numbers aren't keyed directly to the year of manufacture, as some other manufacturers' are but we do have some capacity to link them to a range of years. Now, we would expect a 60s watch to have its serial between the lugs but a 40s watch to have it on the movement. Given the mix of influences here, who knows where it would be! The case looks polished, so any serial between the lugs may not have survived anyway.

Let's sum up. There seem to be a mix of influences from about 3 decades here, and some things that aren't characteristic of other Heuers. You should always ask for pictures of the movement and the inside of the caseback, especially on an unfamiliar watch and we could check those too. At best, though, this would be at best a refinished watch with anachronistic parts, at worst something else masquerading as a Heuer, so I personally would stick with my instincts and walk away. Go for one of the well documented Heuers instead.