I haven't been able to identify a positive application for this insulator, but some Hemingray experts agree that it heavily leans toward being a Hemingray product. It has a six-sided top just like a Hemingray wall tube and the glass's appearance and color strongly suggest Hemingray. The color is lighter than seen in the pic, more of a light/medium aqua. There are inner threads indicating it screwed over a secondary unit of some kind, possibly as a fuse cover of some sort. It was purposely made of glass to provide insulating properties. It stands 2-1/2" tall and is 1-5/8" across the top from one hex flat to the opposite. It has all indications of being made of early glass and glassmaking procedures. I have not seen another, so possibly unique? Great condition, just very minor flaking on two of the hex top points and some minor surface annealing lines. $350 plus shipping/insurance. PayPal or check. Cash at Springfield works, too, with free delivery. (see pic 2)