COMMENT: The affricatisation (š́ > č́) in PWC can be probably explained if we assume a dental prefix (č́ǝ < *T-š́ǝ, possibly of a class origin); dental prefixation is less frequent in PWC than labial prefixation, but several cases of it are attested.
See Trubetzkoy 1930, 277 (at least the comparison of Ub. č́a and And. š:iwu is correct).
