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.

I've been thinking about these a bit lately, and I have some theories...
- The ANSI code is H-140
- The bulb wattage is probably around 125w
- The total system wattage is around 140w

I should dig mine up and play around with my Kill-a-Watt and Fluke 115.

@icefoglights
They have an ANSI code?? News to me! That's be great if you could take some measurements

Total system wattage is more around 250 w, according to the rated amperage.

@Lcubed3
Assuming you mean 2.1A*120V, then yeah that's be around 250VA. This is useful information I will see what I can do with it.

Assuming the discharge power factor is .85-.9 which seems to be a reasonable estimate, we can calculate likely wattages. Assuming an arbitrary minimum voltage of 50V and a maximum of 70V (~60% of 120), we get a range of 50*2.1*.85=89W to 70*2.1*.9=132W, with an average of 110W, which pretty solidly excludes 175W as a possibility. Now 100W and 125W are most likely, though I wouldn't rule out 140W as a possibility.

Total system wattage is more around 250 w, according to the rated amperage.

Since the ballast is an inductive load and has no power factor correction, VA is somewhat higher than actual W.

Ah yes, I forgot about that. I've been running a 150W HPS lamp in it for some time now, and it does not appear to be underdriven.

@Lcubed3
Assuming it still draws 2.1A from the line (which may or may not be the case with a different lamp in it) then the lamp is likely underdriven, as S55 150W HPS lamps run at 3.2A. 2.1A is for S54 100W HPS lamps. Though maybe the current has changed.