Few references contain information on rubber insulators. Gerald Brown's book on nonglass/nonporcelain insulators is probably the lone exception, however, this was published many years ago and is out of print. Even then, there was only surface coverage of the various markings on rubber insulators
There are several reasons for the lack of interest in rubber insulators:
The chart below is the first in a series which provides detail on the subject.
The chart was developed from examining over 50 specimens of the R-4 style
of insulator primarily made by Continental Rubber Works. These insulators
came from a variety of sources including a California collection, and lines
in Canada, Illinois, Indiana, Ohio, Michigan, and Wisconsin.
The column to the farthest left represents the marking on what is assumed to be the crown of the front of the insulator. If the embossing took two lines, the content of the first line is separated from the marking on the second line by a slash (/). The embossings listed here are listed in what I would guess is their chronological order of production.
Column two contains the embossing of what is assumed to be the crown of the back of the insulator. Again, if the marking took two lines, the start of the second line is indicated by the slash. . Note that the third back embossing is an error marking (extra 2 in the patent number).
The third column indicates the marking, if any, on the dome or top of the insulator. Since most of these insulators are marked with "R-4," the nickname for these is made clear.
Column four contains mold number trivia. These mold numbers appear on the crown between the front and back embossing. Each mold numbers for a particular embossing is listed on a separate line. Note that some mold numbers contain unusual spacing (like "8 1 B") while others contain periods in odd places (like ".5-C/3"). Slashes are not part of the embossing but indicate that the embossing occurred on two lines. When mold information occurs on both sides of the insulator, mold numbers on the right are listed first followed by a double slash (//) followed by mold numbers on the left as one looks at the front of the insulator.
|CONTINENTAL RUBBER WORKS / ERIE, PA. USA||PATENT PENDING||R*4|
|PATENT PENDING||R*4||2 1-C,
|PATS 2218497 /
|PATS 2218497 /
8 1 B
|PATS. 2218497 /
RUBBER WORKS U.S.A.
19/3-C 26/3-D 28/5D 32/3-D
|HECHO EN MEXICO (crown arc)||M||.||.|