Information on Oakman Signals, Diamond-P and Other Boston Type Insulators
by Joe Maurath, Jr,

Reprinted from "INSULATORS - Crown Jewels of the Wire", June 1972, page 2

I recently acquired two-insulators of great interest that I think few of us have heard of. These are CD 133 OAKMAN signals. They are very similar to the CD 133 AM. INS. CO. signals. In fact, some of the OAKMAN embossing is visible, although mostly obliterated. I know of very few CD 133 OAKMANS and was wondering how many of your subscribers have heard of them. I have enclosed a drawing of this item. (Drawing Is approximately 75% actual size.)

Also of interest are a couple insulators of a very peculiar design that I believe are of Oakman manufacture (See drawing, approximately 75% actual size, on page following.). I have found three or four, all in this area, and know of only a few others. These are embossed PAT'D AUG. 19, 1890 around the base rim. The patent refers to ridges around the inside of the skirt, as you will note from the drawing. The patent was issued to Samuel Oakman, and, judging from the color of the glass, threading, style of lettering and patent, these were most likely produced by Samuel Oakman.

Most interesting are the "Diamond-P" insulators found here in the Boston area (See drawing, approximately 75% actual size, on page following.). All are marked with a "P" enclosed in a diamond on one side of the skirt, and PAT. AUG. 11. 85. on the other. The patent was issued to James Pennywick of Boston for a method of producing threads in articles of glass (The patent did not mention anything about insulators.)


The manufacturer of these insulators and what the Diamond-P monogram stands for is unknown at this time. Diamond-P insulators are very similar in design to the PETTINGELL ANDREWS CO. and C.E.L. CO. insulators. All three types appear to have been made from the same style molds, and all have very fine screw threading, unusual for insulators of their age, which most likely were produced under the Pennywick patent.

Pettingell Andrews, C.E.L.Co. and Diamond-P insulators are found principally in the Boston area, in small numbers. It seems likely from the area of distribution, and the fact that one of these is actually a Boston insulator (Pettingell Andrews), that they were produced in the greater Boston area. 

Found quite frequently are insulators with no embossing that closely resemble the Pettingell, C.E.L. Co. and Diamond-P  insulators. These insulators also appear to be from similar molds and have very fine screw threading.

C.E.L. Co. insulators have been found in small numbers here, and the exact interpretation of the initials has not been revealed as yet. Some people suggest Cambridge Electric Light Co., which does exist and has for many years. I have not checked the lines in Cambridge for these insulators; however, I have noted numerous manhole covers with the initials C.E.L. Co. on them.

Most C.E.L. Co. insulators are similar in design to the Pettingell Andrews and Diamond-P insulators. However, I have found one that is very likely of Brookfield manufacture. This insulator is very similar to the familiar T-H.E. Co. Items, except, of course, the embossing is different. Apparently C.E.L. Co. had most insulators made for them by the manufacturer of Pettingell and Diamond-P insulators, and a few by Brookfield. Perhaps this bit of information will eventually lead to the interpretation of the C.E.L. Co. initials.

Pettingell Andrews and C.E.L. Co. insulators are rather uniform in color; most are of aqua shades. Diamond-P and their related unembossed specimens vary a great deal in color. Of the dozen Diamond-P's I've found in the past, I have acquired them in eight colors: aqua; apple-green; hazy green-aqua; very bubbly aqua with an opalescent tinge; light green, opague with billions of white bubbles; blue-yellow-green opalescent; light green; and cobalt blue. I know of a few other colors; among these are sea green and light blue. Corresponding unembossed units have been found in some of these colors and in others (opaque green, like CD 145 H.G. Co.'s; rich greens; near cobalt; and In all sorts of blue and green combinations). Most of the unembossed specimens are of aqua shades, as well as the Diamond-P insulators.

By no means does the above information tell a near-complete story. Much information is needed regarding Diamond-P, Pettingell Andrews and C.E.L. Co. insulators. I hope the information mentioned will add a few more pieces to the puzzle, and eventually the story behind all of these Insulators will be revealed.