This thread will have a lot of information... http://www.jalopyjournal.com/forum/showthread.php?t=676427 '55 & '56 axles, axle bearings, bearing retainers & housings are different than the same parts in a '57 axle assembly. '57 axle housing has a drain plug in addition to the fill plug - '55/56 didn't. Starting in '57 (through '64) the differential had stronger webbing in it also.