Left hand treads on the fixed cup with right hand threads on the adjustable cup + old (70's/early 80's) Motobecane frame = Swiss threading!. Not fatal, but it will not be that easy to find a new BB. Phil Wood makes cups for his BB cartridge but it will cost a lot of money. a few NOS Japanese (SR?) comes up once in a while at eBay..........also for a lot of money (close to a 100 bucks just for cups!)........I feel your pain, I have the same thing in my 84 Peugeot PSV.

Good luck building the MB mixte up. look like it will be a nice bike! Hope you find the BB for it soon.

Chombi