I dug about in the Chev. master parts book 29-41 and there seems to be 2 part numbers for the propeller shaft housing and differential carrier 364750 , 364538 with these three numbers after them 37 - 6 and 38 - 7. could those be ratios? I did see a note that the outer has a casting of D or H for New Departure or Hyatt pinion bearings