Ted;
I think you are somewhat beginning to see the light on this origin aspect, but let me add one more little bit. First off I am not a patent attorney & am certainly not up on all aspects of International Patent Laws.
Now "IF INDEED" Darne prevented anyone from building a basic monoblock design after the expiration of Pieper's Patent it would of necessity have been through some "Nuance" of French law & would have been applicable "ONLY" to some other French maker.
Certainly after the expiration of Pieper's Patent the basic concept of a monoblock was "Wide Open" for all.
In fact if Darne was granted a patent on a monoblock itself & not just some modification of it, it would have to be that Pieper had never applied for a French patent & France did not recognise it. Certainly the rest of the World did.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra