Dating Hemingray Insulators

Decoding the date codes

Writen by Bill MeierView Icon Profile

All Hemingray insulators since 1933 have had mold and date codes on them. In the Price Guide, these are generally referred to as [Numbers and dots] and [Number]. There are several different patterns, and some special cases as well.
An "O" was added under the HEMINGRAY on the front. This is actually the letter "O" (for Owens-Illinois) and not the digit "0". However, most people will refer to it as a "zero". No mold number was generally present.

After the "O" was added a 4 or 8, like O-4 or O-8
The last digit (plus 1930) is the year the insulator mold was made. Thus O-4 means the mold was made in 1934, O-8 means 1938. There is a mold number generally under the "MADE IN U.S.A." on the rear skirt.

circa 1939-1940 to end of production
Hemingray realized that having a one digit year wasn't going to work anymore, after the 1930's (sort of like our Y2K problem!!) At this time, they switched to a standard format generally under the "MADE IN U.S.A." on the rear skirt. This format was "MM-YY" such as 23-42. The first number (MM) is the mold number, and the second number (YY) is the year (plus 1900) when the mold was made. Thus, "23-42" is mold 23, which was made in 1942. In some transition cases, the number 9 was used to represent 1939 and 0 to represent 1940.

There are often dots present after the year code. A dot was added for each additional year of production after the mold was made. Generally, these were neatly aligned in two rows after the year code, looking like a row of colons. But, a colon is really two dots. In some cases the dots are around, above, and below the "-" dash. In any case, count ALL the dots, and add this to the mold year (see above). This gives the date the insulator was actually produced.

Mold codes:

The mode code is a one or two digit number, sometimes followed by a letter. In some cases, the letter designated the type of material the mode was made out of. Generally, the mold code is not significant, unless you really are a specialty collector.


In the table below MM represents a mold number (not a month), and Y or YY represents  a one or two digit year. Dots will likely follow the "numbers". This is discussed  in following paragraph.

Style Front Skirt Rear Skirt Date of Mold
Date of Insulator
Mold code
1 O (nothing) 1933 Count the dots around
 the year code and add
 them to the date computed
 in the column to the left.
2 O-Y MM 1930 + Y MM
3* (nothing) MM - YY 1900 + YY MM
4 (nothing) MM - MADE IN U.S.A. - Y
(this format is on many CD 230's)
(Y is often 0, which is 1940)
1940 MM

* Note with style 3, YY may still be a single digit, with 9 meaning 1939 and 0 meaning 1940.

All dots around the year code should be counted (a colon counts as two dots), and added to the date of mold manufacture. This gives the actual date the insulator was produced. Style 3 is the most common.


This is an example of style 3. The front skirt has nothing, and the rear skirt has
        5 - 42 : : : .

MM is 5, YY is 42, and we have 3 colons (each count as 2 dots), and a dot, or 7 "dots". Thus, the mold code is 5, the year the mold was made is 1900 + 42 = 1942 and the insulator was manufactured in 1900 + 42 + 7 = 1949.

Variations and Exceptions:

There are some variations, but they generally can be deduced from the above patterns. In all cases, counting the dots and adding them to the mold year code produces the date the insulator was actually produced.

Some variations include just a 38 or 39 on the front skirt (for 1938 or 1939). In other cases, just the single digit 9 (rather than the two digit 39) was used to represent the year of 1939 (a transition year) as well as just the single digit 0 to represent the year of 1940.