Just so you get the idea I will start the ball rolling with a couple of things I recently found out about the XS650.
Did you know that the XS2 front disk mounting system is different than all other XS's
XS2 Rotor carrier with two indented slots
XS2 Hub with cast bosses which fit into the indented bosses of the rotor carrier
Later disk rotor from a TX - XS650C with six bolt holes for mounting rotor to hub is on the left and the XS2 disk rotor is on the right.
Note that the rotors themselves are identical but the carrier on which they mount is different. I didn't know this until shown by Bruce Cannon when I went to look at some hubs he had for sale on ebay.
This XS2 rotor and carrier is attached to the wheel with a threaded sleeve which slips over the front axle and is held by the wheel spacer and axle nut when they are tightened.
Also on some 256 engines, mostly XS2, the right side flywheel can have lead weights installed in two extra holes as seen below.
This one has had them replaced steel rings and Mallory metal (a heavier option than lead)
Why Yamaha did this is anybodies guess but it could have been an attempt to smooth out the 256 engines which were renown for their vibratory nature.
This failed because the lead weights ultimately became loose and migrated inward to come into contact with the conrod.
There is also a difference between the early crankcases and the later ones in the area of the rear engine mount.
Here is the earlier XS1,XS2 and TX crankcase upper with light casting webs.
And here is the later much heavier webbed upper crankcase.