plastic pole line insulator

Here's one I don't think I knew existed before, until I got it in with the rest of the bucketful of glass insulators from Ohio recently. I likewise don't really know much of anything about it, I'm guessing it would have been for use on railroad type 'signal' lines more than actual power lines, at least any but a simple house drop anyway...???

Shown next to a typical glass insulator for size, it is now sorta milky colored plastic that might have been clear at first. (again, I don't know?) It has small ribs molded on its inside and the name "H. K. PORTER CO. INC." with a part number around its base.