Mark, you uncovered the code I was hunting and now I know that rear was made between 1955 and 1958 and what it is.
I only say what I said because I've seen friends (and I am not guiltless) worry so much about rear ends, transmissions, etc that they nearly became afraid to drive the thing down to a gas station. I've also watched a lot of money disappear out of their pockets into the pockets of repair shops...and never get the improvement they were dreaming about. I also remember a rear end in a 1951 GMC pickup that when I finally looked inside while re-sealing the pumpkin cover was so frighteningly bad I stopped what I was doing. And I'd been driving it all over the place for almost 3 years...many 2 to 4 hour trips on the highway. A friend of mine (a machinist by profession and a good one) told me I needed to keep in mind that these aren't high precision race car rear ends spinning at 200mph back and forth through serpentine curves. This was way before the days of digital pictures, but I described what I found and he said his prescription was to get the thickest rear end gear oil I could find, mix about 10 or 15% of any additive with a high % of zinc d. in it (like STP) and then keep driving until it quit. Then rebuild it. Good luck!
