"AX" , which is the axle ratio, and whether it is a sure grip unit, or not. Here are the codes:

A - 1 = 2.76 ratio
A - 2 = 2.94 ratio
A - 4 = 3.23 ratio
A - 6 = 3.54/3.55 ratio (Dana/8 3/4 axle)
A - 7 = 3.91 ratio
X - blank = non sure grip (single spin)
X - 8 = sure grip (both rear wheels have power)