The version 3 propshaft has an oil gallery drilled into the splined area of the shaft to help oil flow from the inside of the shaft to the slipper clutch. If the end of the shaft was left open, oil and crank case pressure would leak out.
Also, the version 3 propshaft is shorter and does not protrude past the roller bearing at the crank case side. If you put a version 2 oil seal in this position with the version 3 propshaft, it would not seal the shaft and you would have a prolific oil and crank case pressure leak.
