Following the ebay link from an earlier reply by Yamato72 I used the old gravely part number 18031, then looking at the seal and bearing cross reference chart Chip old did however long ago came up with, manufacturer, National, part # 450165.
If you can't find that seal at least you or the parts house should be able to cross reference it to something more current.
Also I can't open it at the moment because I have a new lap top without office of any kind but check the cross reference spread sheet JRD is heading up. It may be in there.
Good luck.
