Advance H37 Mercury Vapor ballasts suitable/recommended for firing the 250w 6500K Iwasaki Metal Halide lamps. The part numbers are links to a local copy of the datasheet for that ballast in .pdf format.

71A35R2 MV 250w H37 50hz 292w@220/240V
71A3502 MV 250w H37 60hz 280w@120V
71A3542 MV 250w H37 60hz 285w@480V
71A3542T MV 250w H37 60 hz 285w@480V
71A3572 MV 250w H37 60hz 285w@120/208/240/277V
71A3592 MV 250w H37 60hz 285w@120/208/240/277V
71A35D2 MV 250w H37 60hz 285w@480V
71A35J2 MV 250w H37 60hz 285w@220V
71A3800 MV 250w H37 60hz 281w@120V
71A3802 MV 250w H37 60hz 285w@120V
71A3825 MV 250w H37 60hz 274w@240V
72C3584 MV 250w H37 60hz 280w@120/277V
73B3592 MV 250w H37 60hz 285w@120/208/240/277V
74P3503 MV 250w H37 60hz 300w@120V
74P3513 MV 250w H37 60hz 300w@208V
74P3533 MV 250w H37 60hz 300w@277V
79W3542 MV 250w H37 60hz 285w@480V
79W3592 MV 250w H37 60hz 285w@120/208/240/277V

These ballasts *may* be suitable for firing standard 250w M58 type Halides (probe start), the only notable difference is the capacitors are rated at 22.5uF for MV ballasts compared to 15uF for the M58's. In my personal tests with a couple of different bulbs on a 71A3572 they fired without issue and the resulting light produced was noticably brighter. If this could result in shorter bulb life has yet to be determined. YMMV