Hey Mark, So I only missed your thread by a couple months over a year, but.... You're correct, it's a 3.07 Ratio and is a 12 bolt. The KAC code means that axle came from a C10 1974-77 & 1979 3.07 non-locking. So the code says it's an open differential, and could have been from a 74, 75,76, 77...