A few people here have the rare Lo-Volt mercury vapor lamp and fixture. The following things are more-or-less known about these lamps:
• They run on a choke ballast on 120V input and are incompatible with other MV lamps
• They have a probe, but that's still not enough to strike on 120 so they use an ignitor as well
• They may or may not use an HPS ballast+ignitor, which may or may not be S54, S55, or others

The following things are not definitively known, or conflicting answers exist on LG:
• 140W seems to be the most agreed upon wattage but 100W, 125W, and 175W have also been mentioned
• The lamp voltage may or may not be 55V (my guess is it's a little higher, but nobody has measured)
• Lamp current seems unknown, I have heard 2.1A mentioned which implies S54 but this is unconfirmed

If anyone has or can acquire any measured data, even if it's just the readings from a Kill A Watt that would be immensely helpful. RMS voltage and current ratings would also be greatly appreciated. Thanks!

I have one of these in permanent service. When we put it up, we put in a 150W HPS lamp, which has lasted for about 3 years now. The model I have is the LC175M.
You can see more about it here.