A barrel of plums, as the name suggests, is full of plums. If it had an apple (or apples) in it, it wouldn�t be a barrel of plums would it!
Therefore a barrel of PLUMS contains PLUMS, approximately one barrels worth, of PLUMS, only PLUMS can be found is a barrel that contains, and is labelled as, PLUMS. NO APPLES. Or it be a barrel of plums AND apples.
The 1st half of the riddle is to throw you off the scent!