Here's another picture which also shows the pin side of this insulator.

I took the liberty of snagging the photo of the insulator purchased on eBay by Jason Townsend and am posting it here.

I'm not a big collector of porcelain, so would someone in the know please correct any misinformation? Thanks!

At first glance, this doesn't look like a very practical design. Wouldn't rain water run down from the wire groove, then around the spiral to the crossarm, forming a fairly short path to ground? Or was this intended to be installed with the wire groove down? Apparently not, as the bottom is concave.

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In the past half day, a number of folks have replied that the spiral groove is intended to capture rain water and thus wash the insulator of pollution and dust.

See: http://www.google.com/patents/US1869397, and click on "description."

In its eBay description, this one was said to have come from ComEd. Pollution? In the Chicagoland area? Well, yeah. Remember that Chicago was a midwestern rail hub, with coal-burning locomotives in use, and also had (and still has) lots of suburban rail service.

Erik Larson's book "The Devil in the White City" states that a thousand trains a day traversed the city. He has a good description of how bad the pollution was in the late 1800s, when each downtown building had its own coal-fired boilers and dynamos. Larson writes, "A miasma of cinder-flecked smoke blackened its streets and at times reduced the visibility to the distance of a single block, especially in winter, when coal furnaces were in full roar." Add to that the US Steel Works and you've got the ingredients for world-class pollution.

Thanks to everybody who replied, including Elton Gish, Shaun Kotlarksy, Dustin Logan, Steve Lohr, and Bill Meier. There were mentions of Jack Tod's "Porcelain Insulators: Guide Book for Collectors," Page 30, as well as ICON's patent reference at: http://reference.insulators.info/patents/detail/?patent=U1869397.