Proto-North Caucasian: *ʔrǟnƛ_E
Meaning: six
Notes: Lak. and Darg. do not have the expected dental prothesis, characteristic for roots of this structure (HRVNCV̄; in Lak. we would expect *t:irik:-, and in PD - *durik:-); this may be connected with old class prefixation or special syntactic features of numerals. Vocalism in PL, Lak., Av. and part of Andian languages (diverging from the normal reflexation of *ǟ) is due to the influence of *ʡĕrŁ_ɨ̆ 'seven' (see below about its influence on the WC form, too).
In WC labialisation is obviously secondary (as in some other cases, it must be explained by former class prefixation: *uV-ƛ́V > *ƛ́ʷV). It is more difficult to explain the lack of expected gemination in *ƛ́: (the EC data point to the final long vowel); maybe this was caused by analogy with the following NC numeral, *ʡĕrŁ_ɨ̆ 'seven' q.v. Still, the common origin of the EC and WC forms is beyond doubt.
See Trubetzkoy 1922, 239, 242; 1930, 275; Абдоков 1983, 155-156.
