Another insulator group consisting of the CD 726 through CD 728.8 is similar to the compromise style. The main difference is a straight skirt rather than one that is concave in shape. The "straight skirt signal" grouping is probably an outgrowth of the compromise style, and most of them date circa 1868-1875. Many of them were produced during the time period when the transition from threadless to threaded types was taking place.
As near as can be determined, the first regular production of the threaded insulators took place sometime between mid-1867 and mid-1868. It is unlikely that large-scale production took place for several months after that time. By 1870, or shortly thereafter, it would seem likely that quantities of both threadless and threaded units were being distributed. By 1875, most contractors would have been making use of threaded units in new line construction. There were, however, unthreaded insulators and pins being advertised by telegraph supply houses into the late 1870's.
The CD 728.5, while it can't be classified as a "regular" threadless since it has a special lock-pin type pinhole, still demonstrates a lingering of "unthreaded" units, and in this case probably dates from the 1880's. A patent was issued to L.C. Baldwin and J.C. Thurston on April 22, 1884, for a special pin arrangement which looks very much like the insulator described above. Some of the embossed signal type threadless also date from the 1870's, but they will be covered more closely in another section of this writing.
The CD 726, the only example of Canadian origin in this grouping, has been found on several routes of telegraph in Canada, and at least one in the U.S. operated by a Canadian company. While some of these units have been found along routes of early line construction, it is possible those were used as a replacement item or as a need arose for the building of another circuit along the same route. Most have been located along railroad routes constructed in the years 1869 and 1870. These insulators are found in some very spectacular colors including at least three different vivid blues, puce, burgundy, amethyst, amber, deep green, and various shades of aqua.