It's so the mounting arm (or part of the upper housing or whatever) doesn't come in contact with the ballast. It's a bit more robust than the paper that's used to cover the coils during the initial ballast assembly.
I think you're right Tony. 750W HPS fixtures are a big chunk of cash as well since they're super uncommon. According to Niall and Joe though, the MTO lieks to use them in their highmasts. I think the few highmasts here are 400W HPS or 400W MV (or maybe they're MH, i dunno).
Yeah, MTO has a good number of 750w HPS highmasts here. I actually saw a 750w HPS GE highmast for sale at $70 here but I didn't get since I didn't have the room and also because the fixture was 347V only.
The 325 I just got from GraybaR was $257.38 and that was the same configuration but 250 watt. these bad boys can get pretty exspesive. And mine deosn't even have the plug in ignitor.