As an owner of a SV-175 American Electric (branded Electripak) fixture, I'd guess this is a Philips Lo-Volt lamp, though I've never seen one with that etch. If it is dim/never warms up on a standard 100w mercury or metal halide ballast then try it on a 55v, 100w HPS ballast. It should get nice and bright and won't cycle. Some of these fixtures apparently actually used an HPS reactor ballast (with ignitor!) but came with the Lo-Volt mercury lamp. And yes, they are 125w; why the fixture was branded SV-175 I don't know...