French-thread pedals and crank-arms - a guaranteed fit?

I'm having some trouble fitting a pair of French pedals (Lyotard 460D) from the '60s into a pair of Durax crank-arms from the late-'50s or early-'60s. Actually, one side screwed into the crank arm pretty easily. The other screws in about 3 turns, and then gets progressively harder to turn to the point where brute force would become necessary (and which I'd rather avoid). Neither pedal has any markings on them (other than the Lyotard dustcaps); no "L" and "R" or "D" and "G". I mic'ed the threaded spindles and they're 14mm, but I don't have a thread pitch gauge with which to determine that measurement. I've put a small wire brush into the crank-arm receiver and cleaned it out; it seems to be pretty clean. The threads on the pedal spindles are clean. I don't have a tap & die of the right dimensions with which I could chase-out the receiver.

Edit: I just tried to measure the threads-per-inch (or centimeter), and it appears to be either 1 tpmm or 24 tpi. That probably doesn't help distinguish anything, but still...

Any suggestions?