3 3/8" X 5 3/8" (83mm X 132mm)   (front skirt inkstamp) OTE                         (rear skirt inkstamp) {galvanized metal cap} {Kloster Veilsdorf}    Picture courtesy Bob Berry

This is a dryspot insulator where the top unscrews to expose openings inside the skirt and wire groove to allow a telephone drop wire to pass through the insulator to provide a "dry spot" from rain and decrease leakage loss. 

U number assignment from Worldwide Porcelain Insulators 1986 Supplement by Marilyn Albers & Jack Tod.