Here is a picture of the assembled unit. Note that this not the piece Jeff Katchko found at the site nor is the Ramshorn the correct style. However, it shows how it was mounted and used. Now, for the best part of all... In the 1877 issue of the Telegraph in America, page 320, there is a picture in the margin. It's easy to see it's a picture of a hook with the glass attached! The description next to the picture reads:
"The pin and glass in one form or another was the accepted style. Almost the sole exception to this was a plan adopted, to a certain extent, in an effort to prevent the extensive breakage of insulators, but which was only partially successful. This was the employment of the hard rubber-covered hook and stem insulator in connection with an inverted glass saucer. The imperfect character of the rubber soon made the insulator worthless."
So, all parts fit together, and we know exactly what this "piece of glass" is. It's the "inverted glass saucer" that goes on top of a hard rubber Goodyear Ramshorn!