I really don't think it is a proof mark.
I have never seen that mark before.

The DD mark is now well recognizable.
Didier-Drevet was a famous barrel maker.
I don't know if Pierre Drevet who was the CEO of the Manufrance starting in 1945 was directly related on not.
Dider-Drevet was incorporated into Verney-Carron along Jean-Breuil in 1963 IIRC.
WC-