Comments: The final glottal stop in Chech. and Ing. is secondary (as also in other numerals). The original base šin- is preserved as oblique (Chech., Ing. šina-).
Cf. also PN *šina-ra 'heifer (about 2 years old)' (Chech., Ing. šinara) which is, on one hand, an obvious derivate of *ši(n) 'two' (just like *qaʔar-ḳ 'a 3-years-old heifer' from the oblique base *qaʔa- 'three') and on the other hand - a perfect match for Lak., PL and PWC forms.