Read Stovebolt link I added above. A real, era correct differential from 1959 is rare. The value of a hypothetical unit is not possible to determine and will depend on the actual year. Rare is not always valuable. In the case, the value is as usual: Condition? Ratio? Where is it? Who has it? How bad do you need it? How bad does the seller need it? Asking price. Sales history. Result: Free to $500. Can't imagine that it would not need rebuilt and if so, where are you going to get clutches and etc. for a 59? I rebuilt a genuine 1960 Dana 44 (open), the parts were listed as 60-up. Chevy and GMC were in a huge transition in differentials at that exact time. Books say one thing, actual truck have anothers. HO's, 44's, 45's, several "10 bolt" styles. The ring gears have ratio and date stamped on edge.
