Is this insulator the rarest of the BROOKFIELDS? 

This insulator has unusual embossing and a patent date that has been listed in various places as April 17th, 1896, as well as April 7th, 1896.

It interested me to the extent of contacting Jack Tod, our hobby's fountainhead* of information. He supplied a copy from the Official Patent Gazette for this patent with the correct date of Tuesday, April 7, 1896. But the drawings and information in the Official Gazette did not resemble the C.D. 134.6. The two drawings did resemble the C.D. 112.4, and I don't think Brookfield made any of these. 

For the first time, I requested the full Letters Patent from the government Patent Office. See these patent pages for the results!

Indeed, the Mad Hatter, made to the modification shown in figures 5 and 6, and described in Hannibal Rappleye's letter of invention?

It should be noted that the Official Gazette did not include figures 3 through 6, nor any mention of a modification.

In Milholland's photo I can't discern any projection such as #100 in the figure 5 patent drawing; but if the insulator does have this feature, it would likely necessitate the use of a two part mold.

The following are just some personal opinions. 

It seems odd that this insulator was not embossed with a W. or Wm.; but then it may have been some number of years after the patent of 1896 before it was actually produced. 

Also, the Philadelphia embossing suggests a localized and limited area of use.

The insulator would be costly to make, and costly to sell, and very likely unsatisfactory in use. So maybe Brookfield threw this mold with their name on it away. At any rate, the Brookfield catalog of 1912 does not list it; whereas the unusual C.D. 119 O'Brien and the odd C.D. 139 Schoethaler patent insulators of the 1880's and earlier 1900's were still cataloged. 

So unless some dude has preserved a bank vault full of these jewels, I personally consider the C.D. 134.6 to be the rarest of the Brookfield insulator manufacture.

* Jack -- Completely complimentary, as opposed to a nickname.