I have found the oil fill on the front of the superchargers but not on the back. I have looked and looked. There is however a brass plug on the top right side (looking at supercharger from front) on the supercharger. Is that the other fill hole? There is a dipstick on the one in the top front of the supercharger but is there only a fill on the side(if it is the fill)?

There is no oil fill in the back. Sealed bearings in the rear plate.

There is a fill hole on back of SC. It should be an allan head plug.

I agree with dykstra, dip stick in front, allen head plug in the rear. both front and rear require oil. I changed mine and used mercruiser synthetic drive oil in them. They ran great until I sold the boat.