The adapter on the left in the above picture will allow you to use a standard Parker ballpoint refill in a section originally used with the soft-tip or felt-tip section. Without this adapter, the writing point of the BP refill is too small for a proper fit in the section. The plastic adapter on the right is used for the closed end of the BP refill to give it the correct length inside the 75 barrel.
As you can see from the above package, it was the same one used in the plastic Big Red pen. Many times, you can find them at pen shows by simply buying the plastic Big Red BP.