lighting question s50/m58

[ls1boost]

i have a diy 250w metal halide system over my tank it has 2 250w magnetic ballasts recently the capacitor went out on one of them. it was cheaper to buy the ballast and capacitor together rater than just the cap. i recived it today and it says use with s50 lamps the person that i bought it from said that he used m58 bulbs are they interchangible i wanted to ask before blowing a bulb thanks in advance