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 
companies. 

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.