Two of the earliest dialectal divisions among Iranian indeed happen to not follow the later division into Western and Eastern blocks. These concern the fate of the Proto-Indo-Iranian first-series palatal consonants, *c and *dz:
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What elements of Proto-Indo-Iranian did not diverge according to the ensuing split between eastern and western variants?
*c and *dz