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This page needs: Dating and ID info for other manufacturers, manufacturer initials, info on insulator types

Insulator Identification and Dating

It is common to want to know the origins and age of an insulator you found. This guide will explain how.


First off, determine the type and manufacturer of your insulator. Examine the outside of your insulator for any obvious names or initials. Common initials include:

Initial Manufacturer
A Armstrong
B Brookfield
(“B” inside an “O”) Ohio Brass
(“I” inside a diamond, overlapping a large “O”) Owens-Illinois
WT Whitall Tatum

If there is no obvious manufacturer name or initial, see If That Fails.


Look at the insulator and try to find a date. Usually, it is pretty obvious. If not, look for any numbers not accompanied by a company name. For example, a single 57 may represent 1957.


Look for a circle with an A in it. If it is followed by two numbers, the first number is the mold number and the second number is the year +1900 the insulator was made. For example, (A) 39 49 means the insulator was made in 1949 with mold #39.


Unfortunately, dating Brookfield insulators is not an exact science. However, if you follow this guide you can get a good idea as to when your insulator was made.

If your insulator:

  • Has [[ drip points ]], it was made during or after 1909.
  • Is light aqua or green, it was made in Brooklyn, no later than 1908.
  • Is dark aqua or green, it was made in Old Bridge from 1906 to 1921.
  • Is light blue, it was made from 1915 to 1921.
  • Brookfield made insulators from 1906 to 1921 when they went out of business, so your insulator was made no later than 1921.


Later Hemingray insulators have a system based on 1930. On the back, there may be a symbol like O-3::.. The O- can be ignored. The 3 means 1930+3, and each dot after is an additional year.

If That Fails

If this guide did not work, see this website. At the moment, it is much more thorough than this guide.