DH, Here a copy of what someone wrote in a discussion about this. I wouldn't have known about this hook up myself if I hadn't run into it and had to figure it out to get the lights working again.
Up until somewhere in the 1920's, there was no Code requirement that the lampholder had to have the neutral on the screw shell. One will find examples of this "Carter" 3-way in electrical inspector approved installations from the first decades of the 20th Century.
Many two story homes that were wired with Knob and Tube had lighting on the main stairs that was controlled by a 3-way switching setup.
One of the 3-way switching hookups that was used was this polarity-reversing method.
One brought a hot and a neutral to each switch location and hooked them to the switch traveler terminals, and then sent one wire off to the lampholder from each switch. There was a real economy here, for the installing electrician.
I have found that many times, on little two story homes, that the hot and neutral came from different circuits, on occasion. This is a particular delight to discover on a service upgrade, when one takes one of the two circuits (both should be on the same service leg) and inadvertently puts it on the "other side" of the panel.
When the light should be OFF with both the screw shell and the center button HOT, suddenly, becomes a 240 volt bolted short circuit.
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