The simple answer is that John C. Barclay's patent included two versions in the patent drawings. One is the style we are familiar with today, with the spiral being indented into the glass and with a standard indented wire groove.
The second is an example with raised protections forming a dash pattern around the insulator where the wire would sit between.
The drawing in the early guide (left) is contrasted with the second patent drawing (right). Basically an exact match including placement of the "dashes"...
The patent information can be found here:
I suspect, but we will never know, that this was an insulator that never existed, but presumed to exist because of this patent drawing, and thus included in the guide . Here is a link to the [id=467129498; original page in the guide book].