Yes, it is an OV-15 with a 400W MV ballast. They offered them in 240V NPF reactor or 120V NPF HX ballasts. No capacitors, since they'd fry in a small fixture. Modern capacitors supposedly would last a little better (or manufacturers just don't care anymore...).
Mike, how well does a 400 watt MV do in one of these? Have you seen an OV-400 in service? I thought about using this fixture indoors when I get my house instead of outside so it doesn't weather. I want to use it but at the same time I want to keep it NOS. But I am concerned about ventilating this fixture properly.
They hold up OK. MV doesn't suffer from severe heat like HPS does. There's no capacitor inside for that very reason, since these things are like engine blocks after they've been running for a few hours (especially indoors!). If you do install it indoors, I'd install a fan next to it about five or ten feet from it where it can blow air over the top of the fixture. the blowing air will carry heat away from the fixture.