Good question. Let's go through the timeline:
- 29 June 1985: TAG announces the acquisition of Heuer from Piaget/ Nouvelle Lemania
- 1 January 1986: Deal is completed and the name changes to TAG-Heuer (later TAG Heuer)
- March 1986: TAG Heuer show the Formula 1 series at the Basel show
So the question is whether the Formula 1 was developed under Heuer or TAG Heuer. Seeing the "Heuer" crown on your watch means that it's possible that it was developed by Heuer...after all, 9 months is not much time to commission, design and launch a new watch.
To counter this, the reality is that Piaget never put any money into developing Heuer, so it would be a surprise that they invested in the idea.
This Heuer/ TAG Heuer confusion lasted well into the 1990s. Take a look at this ad from 1990:
The watch is called the "Heuer Formula 1" despite the name having changed four years earlier. Bizarre.
Given development time, my guess (and it's a guess) is that at least some of the work on the Formula 1 was done before TAG arrived.
If that's right, then the first "pure" TAG Heuer would be the S/el from 1987.
And yes, you did see combinations of Heuer and TAG Heuer branded parts on watches for some years- rather than scrapping the parts with the old brand, they used them until stock was exhausted.
