OB question for the group. Starting in the 1970's a manufacturing date (month and year was added to their markings. The small indention added to the date sometime in the mid 60's and signifies tie insulator is compatible with preformed tie wires, Picture is a U-320, manufactured inOCT.1981 and i was wondering how many other OB pieces (u numbers) with this date code are known. Would love to see picture's if you have one or two or? Info about markings from Porcelain Insulators, page 134.