Apparently English notch arises from a word-division error due to the English movable n: it wasn’t originally a notch, it was an otch (cf. French oche “a notch”). So says the AHD and the tyrant OED.
This is analogous to English adder which was originally nadder (from OE nǣdre “snake”), but people misheard a nadder as an adder.
This movable n in English might be more trouble than it’s worth, but it’s probably too late to put a stop to it.
