Some semi-random thoughts and apologies if someone else has already thrown this out there...
Continuing on from
Kurt's excellent info on how to spot a Val. 72 806:
Val. 72 pushers on the left (from Kurt) vs. Venus 178 pushers on the right:


And we can see here what Kurt and others are talking about with the telltale asymmetrical pushers:

But this also means that the cases would have had to have been
drilled/machined differently to accommodate the different spacing of the pushers.
Then again, with the 806 being a new model anyway with a bigger case than the Chronomat, would this really be such a big deal if Breilting went away from their usual chrono movement supplier?
Also, I'm not sure what this means regarding chronology, but it could again point to some sort of interruption in the flow of Venus 178 movements that forced Breitling to both contract Valjoux movements as well to produce special specs for their case maker to accommodate them.
Now, whether Breitling produced some Venus 178 806s
first and then made the brief switch to Valjoux movements due to a shortage/technical issue (lack of Incabloc?); or whether they
started with the Val. 72 movements/cases and went back to their "normal" supplier Venus with a different case for the 178 for some other reason (cost, maybe?)....well, that remains the 100,000 Swiss Franc question!
Best,
T.