This photo of an SS 10 D CD 638 insulator is also from the Lviv Insulator Company's website, as described in my previous posting about the NS 18 A. The factory is located in Lviv, Ukraine. The Cyrillic letter for "SS" or "ShS" looks like a capital letter E with the "prongs" pointing up, the D looked like a stylized "delta". I don't think that I can import Cyrillic letters into this format.

[IFONA CD 638]