The M-58 is indeed a legit Metal Halide and it is 250 watter.
If you see the " H " lettering that is old mercury vapour that is pretty much history like dino.
The Metal Halide bulbs typically run little hotter than mercury vapour is and they are pretty senstive to burning postion as well espcally with hortzotal verison.
If alot of stuff look pretty much burnted or about ready to become FUBAR it is good time to ditch it and get new one.
15-20 years is not too bad but little short side on lifewise if properly maintened it should last at least 25 years before something major screw up.
The other thing I am not sure but if you can read the ballast info you should able tell if that wallpack have proper bulb in there with med et larger wallpack it common to see 250 and 400 watter in the same housing.
some case I know someone stick in 250 watter in the 400 watt ballast that can overdrive the lamp and really shorten the life and some case it can exploded.{ for sure you will heat up the whole thing pretty easy }
Merci,Marc
