Hi,
the genuine twinport and sidepipe tops and bottoms are the same,different steering head stem length, 1 inch shorter, than the later ones(see parts book) the yellow tanks, black frames have the same internals but visually different bottoms, red frames have different internals, damper rods, top out springs,guide bushes,are thinner at the top and no bronze bushes in the sliders,75's have external spring load adjusters at the top and double bolted lower fork clamps and longer springs,tubes and damper rods but shorter top out springs.
Changing the springs, damper rods,top out (rebound)springs and guide bushes,on the earlier forks you can get a real 7" travel or more if you aren't careful,
As you can see they are all look the same, except for the bits that are different !
cheers