This post in shows the backs of three different CD 133.4 that were found on the Chicago Northwestern Railroad as it ran East out of Boone, Iowa toward Ames, Iowa. Details are as follows:

CD-133.4 Patent /Dec.19 1871 (Front Dome) - Large Script 3 (Back Dome - No Mold Mark) - Medium Green (Large Bubbles / Considerable Milk Swirls) - Measurements: 70 mm Wide x 98 mm Height.

CD-133.4 Patent /Dec.19 1871 (Front Dome) - Medium Script 3 (Back Dome - No Mold Mark) - Teal Blue (A Few Bubbles /Microbubbles / Crude Overpour / Cull Stones) - Measurements: 69 mm Wide x 98 mm Height.

CD-133.4 Patent /Dec.19.1871 (Front Dome) - Small Script 3 (Back Dome) - Mold Letter "A" (Back Skirt) - Light Celery Green (A Few Bubbles Microbubbles / Very Crude Overpour) - Measurements: 67 mm Wide x 97 mm Height. Also note that the threads of this insulator are off-center.