French thread pedal question
 

Ok. I'm parting out a Bertin with a damaged fork and want to verify that the pedals are french thread. A french thread pedal will loosely thread into a conventional 9/16ths arm, right? It just doesn't work the other way. I know better than to try any more than loose finger tension in trying a normal pedal in the frenchy crank arm, but I'm just double checking.

You are correct:
You can install a French pedal into a standard crank - but it will be too loose.
You cannot install a standard pedal into a French crank.

This is another reason i never bought a french bike back then... Oops, wait, I did buy a Motobecane Le Champion way back, but it had English Campy NR pedal threads. French BB threads though, I Still have those BB cups...

French threaded pedals may be marked D on the right and G on the left.