I can't see any reason why a Heuer bridge would end up in an original Breitling. Yes, there were some parts shortages initially, limiting the number of watches the coalition could get out the door but I doubt signed bridges were a particular issue, especially not by the time Breitling were making the Pupi. If anything, if a Breitling bridge were to be signed anything other than Breitling, I would expect it to be Kelek thanks to the manufacturing and supply agreements Willy Breitling had with that firm.

Willy Breitling and Jack Heuer had reached a gentleman's agreement over Heuer not using the "Chronomatic" name on the Heuer dials, it would be in pretty bad faith if Breitling then went on to use Heuer-branded movements!

So I would lean towards that being a donor movement from a Heuer at some later point - the date wheel looks correct for the Heuer.