I now have two ITT H37-5KC/W 250 watt mercs. I have a few questions:
How rare are they?
Who is ITT?
Are they affiliated with ITT tech? Logo comparison included:

ITT: International Telephone and Telegraph, founded 1920.

They became a corporate conglomerate buying up anything they could get their hands on. At one point, they owned American Electric, later acquired by T&B (Thomas and Betts).

Wikipedia has a pretty juicy article of their involvement with international politics and other shady scandals.

ITT Mercs aren't anymore rare than any other Merc from that time.  Growing up, my neighborhood had ITT Mercs and GE Mercs, and only 4 HPS fixtures existed.  Of course, by now, there may be 4 ITT mercs left and 2 GE Merc fixtures.  So I would say that any ITT Mercury Luminaire you find is probably pretty rare considering their age.

I've seen ITT fixtures around the older parts of town that are still Mercury vapor, still used, and still maintained. One got replaced with HPS recently since a car took a pole out, sadly. 

Not sure about Mercs.. but one of my church buildings had a few ITT F40T12 fluorescent bulbs dating back to when the building was built in 1972.. I brought them home and most did not last more than 100 hours before going EoL so they had been heavily used.