Here's how the codes break down; the Letter part of the code give the year, ratio, and plant, the Numbers are the month/day. In the case of the first diff, it is a '55 1rst, buit at Detroit Gear and Axle, 3.90 ratio, and was buit on December 21. The other axle is a little fuzzier on the year, as the same codes were used from '55.2 through '59. It breaks down as a 3.90 built on August 6 at Detroit Gear and Axle.
