The K72 series trans axles in the X5xx series use similar castings but the X500 (K72B) has no internal passage to route the oil through the replaceable filter you think is missing. you can put the filter in there but it will do absolutely NOTHING. The X500 K72B has an internal filter but changing it requires trans dis assembly.

All the X5xx transaxles have the round cover where the filter would go but the only models that can use the filter (K72A and K72C) have hydraulic lift and power steering.

You can go to Tuff Torq's web site and look at the exploded view of the K72A/C and K72B.

Thanks, Thanks makes me feel better