I seem to recall seeing small grooves or channels in the breech face of some doubles extending from the centers of the firing pin holes at about 30 degrees above the horizontal, to the edges of the breech face. I believe one was an Ernst Steiglitz 9.3 X 72N.
I'm with you, there is a need for some gas relief, I will probably vent my double Francotte 16 if I convert it to a rifle. It needs striker bushings also.