The bulb you originally bought was mogul base or the fixture was? Those should both be medium base fixtures. Either would work, but if you can find a version without that guard piece it'd be better since you'll want the light close to your plants. Also, those don't cheap fixtures generally don't do a great job of reflecting light. For just a couple bucks more you could get a better clamp reflector from a pet store with a similar reflector that is painted flat white inside and a ceramic socket. You could also make a basic aluminum reflector for less, depending on how you plan to use it.