Nathan - I was able to do a little more quick research on your question. The below screenshots are taken from pages 56 and 57 of this
1959 Chevrolet Truck Engineering Features PDF that provides a fairly detailed description of how you can externally/internally ID the Positraction rear axles beginning in 1959.
I did further searches through the PDF's in the GM Heritage site and found that for C10 Model 1/2 tons from 1960-62, the 2 letter prefix on the rear axle serial number sequences were either MC or MF. Both of those would be followed by a 2 numeric digit number indicating the day of the month built followed by 1 or 2 more numeric characters for the month of the year built (see 3rd pic showing 1960 info). The 1959 data file is missing the serial number listing info so unable to confirm those details. Good luck in your search.
