I recently picked up a few of these glass aerial spacer insulators. They are 9" long with wire grooves at both ends (and in the middle on two of the pieces). They were used as spacers over long spans to keep the wires separated. Conversing with one AT&T lineman who painstakingly recovered a handful of these back in the late 1960s, once they were up in the air it was extremely difficult to get them down without breaking them due to how and where they were typically installed (over canyons, washes, etc.)

The one on the left is unembossed. The one in the middle is embossed "E_50" and the one on the right is embossed "E-50". The stamped embossings perfectly match my 1930s Hemingray insulators, so I'm almost certain they are Hemingray products. Bob Stahr said he has a portion of one from the Hemingray dump and believes they are also likely Hemingray.

Embossing detail: [id=631150750]

My question to other collectors: have you seen more of these? Different embossings, sizes, colors, styles? I'm trying to gather more information on these but I'm striking out. I know there must be more out there in the hobby!