It would appear that a friend will be giving me some "scheduled replacement" highbay lights with
S51 ballast's on them (in those nice prepackaged metal boxes with just a positive and negative hanging out). My question is what bulb can I run on it? I know the S400DD used to be the standard but I was wondering what other options I might have. Thanks!

BUMP

You would want to look at other manufacturers conversion lamps. There is a demand for lamps that will give white light on a sodium ballast so industry can make the changeover cheaply.

[edit] Hidirect.com has a venture lamp that will run on that ballast... but it has a 35.4mm arc and an ed28 lamp form.

So the USHIO S400DD (if i can still find one) is pretty much the safest bet?

I've been relying on the words "retro fit", "hps conversion bulb" when finding a lamp to test. Sometimes they do mention that the bulb is actually and S51 HPS bulb.