The sibilant is original ś hence spelled with ש in early and some Jewish dialects, even in derived forms and in the older Syriac inscriptions and documents.

The emphatic plural in Syriac means "tens" as opposed to the absolute ˁsryn which is always "twenty." See s.v. ˁsryn num.
In poetic language in Syriac (as regularly in Mandaic), the "teens" can also be express by "ten and ..." after the manner of the numbers higher than twenty, etc.