The 7.7K shafts have the thinnest shaft and the boots have four accordion folds. Also, the bearing cups at each end are the smallest and look round.
The 10.3K shafts are a little thicker diameter, the boots have maybe twice as many more folds and the bearing cups are bigger but still pretty round. They have the same number of splines as the 7.7 shafts.
These two are interchangeable.
The 12K shafts have even bigger bearing cups and they look more triangular. Also they have more splines so they will not interchange with the lighter shafts. If you have heavy shafts you have to have heavy hubs.
Look in the TM's, there is great info about rebuilding half shafts with exploded pictures of everything.
Augi