The O in FO means orthochromatic. It can be processed with red light.
BTW the 5 means gradation 5.
I know. Just thinking... for gaining a Sabattier effect, you need to expose the film to the actinic light while developing. Apart from a sudden bright idea that might have came to my mind, only safe light was in place - and the effect on the film is reproducible, while bright ideas are very rare. Anyway, there is no trace of fogging on less-/under-exposed areas.