The Ub. form with glottalisation suggests PWC *ṗ-; the PAT reflex *-z- confirms it (if in PWC we had *bǝć̣A, then even after assimilation PAT *-ź- would be expected; on the other hand, *-ź- normally yields PAT *-z-). However, earlier glottalisation must have resided on the affricate, since the initial labial certainly is a historical prefix and there were no glottalised labial prefixes in NC or WC.