When making reference to the "compromise" insulator, the shape that first comes to mind is that represented by CD 731. While this was probably the most widely used and largely produced insulator of the general pattern known as the "compromise", other similar varieties exist. They include all the examples in the series of CD's 729 through 732.2. This grouping has the general classification among collectors of "concave skirt signals". Before going further into a discussion of those items, perhaps it is best to study the following information written by Frank Pope for an article that appeared in The Telegrapher in 1871:
The pattern of insulator shown in FIGURE 13. was designed in 1865 by Mr. M. L. Wood, General Superintendent of the United States Telegraph Company. The internal screw thread shown was a subsequent invention, patented by L. A. Cauvet in 1865. This model was called the "Compromise" insulator. It was intended as a sort of cross between the "egg" and "umbrella" insulators, and appears to have been designed under the very prevalent but entirely erroneous idea that the latter was theoretically the best form of the two, but that, to obviate too great a liability to fracture, it was necessary to approximate some-what to Mr. Swain's pattern. Mr. Wood afterwards improved the arrangement very much by having a collar turned on the supporting pin, nearly filling the mouth of the insulator. When the Western Union Company took possession of the United States wires in 1866 they inherited this insulator and between that year and the present one that company has probably put up between one and two millions of them, carefully discarding, however, their only redeeming feature, the protecting collar on the supporting pin.
The Atlantic and Pacific, Pacific and Atlantic, Franklin, and other competing telegraph companies, who are always very careful to copy with ludicrous fidelity all the electrical blunders of their great rival, also adopted this model of insulator for their own wires.
(Figure 13.) The "compromise" insulator.
The insulators were manufactured by various glass companies, saw very widespread usage, and are among the latest of threadless manufactured in this country. While they have been located in recent years throughout the eastern and central states as well as several western states, one right-of-way in particular has probably produced more of them than any other. Many were found along the route of the Union Pacific Railroad in Wyoming and Utah which was constructed in the late 1860's. They were primarily the CD 731, both unembossed and with the "S. McKEE & CO." embossing. Most of the ones along the UPRR right-of-way were used by a commercial telegraph company and were placed on poles on the opposite side of the track from the railroad company's line which made use of the CD 735 Mulford and Biddle.
The CD 731 was also largely used on a line connecting the eastern seaboard with the Maritime provinces of Canada. Many of the unembossed CD 731 as well as the CD 731 Tillotson have been located along a telegraph route between those two areas serviced by the American Telegraph Company, which was in turn taken over by Western Union upon the merger of the two companies in June 1866. The above-two-mentioned locations where the CD 731 saw extensive use are only a couple of the many areas where they were employed.
Of the other known insulators of the compromise pattern, the CD 732 and CD 729.1 are probably next on the list of availability. Most of the CD 729.1 units have been located in southern Pennsylvania. The CD 732, while not a common threadless type, has been located in various eastern states, primarily Ohio, Pennsylvania, and New York. Of the remaining compromise types, most are scarce or rare and have been found in scattered locations, once again mostly in eastern states.