Drive shafts

Any one know why there are two different drive shafts for T80? One has a shorter 'push screw' (the helix splined part that fits into universal joint) part number 22F-163410-01. The overall length of shaft is the same (although they dont look so in pic as one is nearer the camera) & the spring fixing is further along the shaft. I assume there was a modification at some point. The drive shaft assy (22F-46171-01) is the same for T50 and T80
 

Having spoken to a Yamaha dealer it looks as though the shorter helix shaft is from a early T50 or T80. apparently the shaft was modified with a longer helix bush on later models. Cant see any reason why it cant be used on later bikes.