I think it all goes back to 1935 when Decca distributed the Ultraphone series
of recordings by Django. As Decca did not have it's own studios in Paris at
this time it was logical to use the new Polydor studios when they wanted to
make recordings for their label for distribution in UK etc.
As Decca almost gave up recording any classical music in the UK and had all
the master from German and French Polydor to choose from, there would have
been then have been a very close working relationship between the two
The masters recorded by Django would also have been processed by Polydor for
it would have been difficult to transport these without damage, I presume that
they were allocated an HPP matrix as with all recordings that which emanated
from the Paris studio. The copy master would then be sent to the UK from which
stampers would be made just as happened with Polydor masters for classical
works issued on the UK Decca Polydor label.
Decca would have paid Polydor to do Django records and the allocation of the
matrix was just a byproduct of this process.