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:

1. Limited number of styles.  Continental Rubber Works catalog four styles of rubber insulators: The R-3, R-4, R-5 and RC-1.  An unmarked, unattributed slot-topped rubber insulator can also be commonly found.  Some early hard rubber pin-type insulators made around 1861 exist and ruber was used in making several styles of hooks.  This brings the number of styles to around a dozen
2. Boring colors.  It is no wonder that glass insulators are so popular with their range of colors that can be backlit. Rubber insulators, on the other hand, are exclusively black.
3. Excessive supply.  Continental Rubber Works made 50,000 rubber insulators per year in the 1970s with production probably extending back to the early 1940s.  Since these are more recent insulators, they tend to be the ones still on the poles when they the poles came down rather than being the ones that were carted away to the landfill years ago as some newer style replaced them.  The exception to this is the earlier hard rubber and hook styles which are rare.
   

• Patents by Nelson and Charles Goodyear on the process of making rubber.  These include patents #3,461, #3,462, #3,633, #4,005, #4,099, #8,075 issued May 6, 1851 and #25,111.  The May 6, 1851 date has been found on a hard rubber hat style insulator.
• Patents assigned to The Western Union Telegraph Company under which Continental Rubber manufactured their insulators.  These patents include #2,218,497 issued October 15, 1940, #2,304,483 issued December 8, 1942, and #3,061,667 issued October 30, 1962.
  

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.