I have a John Deere X730 w/54" deck. On line lookup says the spindle is AUC13151, $328.00. Went to my JD dealer, he looked it up and said I need AM14424, $140.00. I compared both and they look the same. Same hole pattern for bolting on the deck, same spindle shaft, same bearings, same blade bolts, etc. why such a big price difference???