The design has a long history right back to WW2 Hanhart flyback chronographs German pilots were fond of. Post war the French, Italians and Germans took up this two register flyback chronograph and a bezel with gusto. The case design of the Heuer(which they inherited from Leonidas) goes back to the mid 1930's as a method for waterproofing chronographs(and tank style watches), whereby the movement is in its own case held into the main case by four screws. The Gallet Flight Officer of the late 1930's has this design and the bezel and was aimed at pilots. 

Interestingly Jack Heuer's autobiography doesn't mention them(I've never seen them in their official history either). Odd considering it would have brought in much needed funds as part of military contracts and is well known in the hobby. Maybe because it wasn't their inhouse design? They were never officially available to civilians and I've never seen one in a period Heuer catalogue. 

For collectors they can be a bit of a minefield. Because the military issued ones were constantly serviced new parts replaced old over time. When SINN took over the maintenance contract for the German military they did similar and then sold off their surplus in the early 90's.

The different dials are down to service updates. The first series have a small T, later they got the red (3H), but because that's the most sought after it's also a bigger minefield where the red ink might still be wet... Then came the service dials with cutoff 8/10/2/4 numerals and then the non tritium luminova dials. A few even got SINN dials. Another issue is because there were so many spares around for these some were building brand new examples from such spares. I've seen case numbers on these go up to 50,000 and I seriously doubt they produced that many watches and IMHO these are more likely to be spares specials. 

When I bought mine many moons ago this concerned me so I looked hard for an early serial, small T dial that had some wear to it, and the one I finally found was a civillian example(no 618), never issued. It was actually cheaper at the time because of that, but I felt and feel it more 'honest'

I've this early 1970's French Dodane cockpit clock. It originally came from an Irish Air Corps Aerospatiale Alouette. It's in dire need of a service and the chronograph button is missing, but it still works and keeps good time. I believe the family owned Dodane company will service and restore these. They supplied clocks and wrist chronographs to the French military for decades. 

Very nice. 👍😊 It could well be a Spitfire clock alright. That type were fitted to lots of British aircraft at the time. Usually on the back you'll find Air Ministry(A.M.) markings and a date.