Such insulators were mostly used on urban telephone lines prior to 1917. The left one is a 1900's M.S. Kuznetsov one, the middle is unidentified (mfd. in 1911-12) and the third one was mfd. by Kornilov Brothers in year 1900. As you can see, Kornilovs' insulator is stamped with both factory mark and the comprehensible date which is a rather rare occasion in pre-1917 insulators. Most Imperial period insulators (especially "E's" and Kuznetsovs) are marked with only one digit referring to their mf. date and the only way to know for sure is to analyse the history of the place they served at.