What do "O" and "L" mean on a lightswitch?

I was trolling around on Ebay late at night a while ago, and somehow wound up looking at old-fashioned US-type light switches, the kind with two pushbuttons (one for on/one for off). Many of them, particularly the Leviton brand, seem to have the buttons permanently marked "O" and "L":

[Picture one](https://i.ebayimg.com/images/g/bRoAAeSwvdFps7Fe/s-l1600.jpg) [Picture two](https://i.ebayimg.com/images/g/7EEAAeSwMj5oYp09/s-l1600.jpg) [Picture three](https://i.ebayimg.com/images/g/cRoAAeSwwNlps7Fe/s-l1600.jpg)

All I could think of was "Light" and "Off", but that isn't it -- I got one and tested it, and it's live/on when O is pressed, and dead/off when L is pressed.

And it's definitely an L (not a I or a 1) and an O (not a 0), plus these switches were in use decades before the 1/0 or I/O convention for on/off.

Any ideas?

Thanks!