I got this Philips 350w for $1.50 at the ReStore in Montclair, CA. I dont have a ballast, but I tested the arc tube on a SS NST and started to light. I has to operate "base up".
No its not a retrofit, its an actual 350w system with its own ballast. PSMH in the USA comes in 50w, 70w, 100w, 150w, 200w, 250w, 320w, 350w, 400w, 750w, and 1000w.
All the Best
Colin