Middle Chinese syllables with the shape CVP where C is a bilabial stop and P is a bilabial underwent dissimilation to CVF, where F = P[-labial]. For instance, Baxter's reconstruction of 法 is *pjop, 品 ...
Schuessler's reconstruction of the Old Chinese reading of 二 is *ńźi. It seems there is a correspondence between OC syllables beginning in *ń or *ńź and Mandarin syllables romanised as er. However, ...