The M designation is for the wattage and bulb type. A M59 is for 400 watt Metal Halide H33 is for 400 watt Mercury vapour.
You really need to know the bulb you are wanting to match. Unless you are using double ended bulbs or HQI bulbs the M59 balalst will drive almost all the Metal Halide aquarium bulbs.
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-=Bryan=-
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