Almost all americal HID ballasts have ANSI ballast codes on them, whether it be M59 for 400W MH, H38 for 100W MV, or S55 for 150W HPS among others of course. But on the etch of many lamps (especially MH and HPS) I see a lot of them with an "E" suffix, like M59E or S51E or whatever. What's it mean?

 Means the lamp is for enclosed fixtures only .

Really? I would have thought that would be in the part number (like LU100E or something as opposed to the ANSI code). Also aren't pretty much all HPS lamps rated for exposed use?

That's what I thought as well. The E denoting enclosed lamps was in the lamp part number and not the ballast code.

Isn't it so when the ballast drives the lamp in a way that may make the explosion prevention features ineffective?