This is one of the best junk filled fizzy Diamond Groove Chicago Insulating Company CD 135 examples I have seen and is shown on the left and a top embossed "PATd" version with over an inch of dome glass is shown on the right.

Base embossed Chicago's are thought to be made by Brookfield. This is because Brookfield advertised this type in an early catalog, and an article with the following information was found by Bob Stahr:

Electrical World New York, NY, United States, Saturday, March 16, 1889 vol. 13, no. 11, p. 166, col. 1

OFFICE OF THE ELECTRICAL WORLD 168-177 POTTER BUILDING, NEW YORK, March 11, 1889.

The Fiske & Mott Insulator, which was illustrated and described in your columns two or three years ago, is now being made and sold by Mr. Wm. Brookfield, 45 Cliff street, this city. It is a high-resistance insulator, useful for all classes of service. A minimum contact surface is offered to the wire through the simple, yet effective device of placing in the groove six ribs on which the wire rests, so that without any loss of sustaining power the total contact is only about half an inch. Evidently there is a vastly higher insulation resistance on the line wire.

The reason I believe that Brookfield made the base embossed units is because they are made considerably better then the crown embossed types. The molding process though is still a Brooke's style which may have been authorized through the Chicago Ins. Co. who controlled the Fisk and Mott patent and apparently had rights to the Brooke's molding process. (both base embossed and dome embossed types share the same plunger method of manufacture).

Brookfield was assigned the rights to the National Ins. Co. "Cork-screw" design and these are found with base embossing and National Ins. Co. embossed on the base. This fact can then be possibly attributed to the Chicago Ins. Co. based embossed units.

This is only an opinion and my suggestion as to the origin of the base embossed Chicago Ins. Co. insulators.