If the tractor "has an inner filter" you should have seen another wingnut
that would be holding the second filter using the same threaded rod as the out filterer.
The inner filters mut be replaced every time you change the inner filter as the inner filter is the last defense protecting your engine as it is much finer filtration.
I would prefer the oil bath air filters as they do not clog and the oil that is in the pan holds ALL the dirt and the engines last much longer.
leon