72 is a one year rotor, no bolt holes. 73-76 have 6 bolts and are two piece with an aluminum center that the rotor bolts too. The one pictured above looks like a 77 and later rotor. The offset is different so they are not a direct fit.
Front XS650 rotors are 320mm Rear rotors and XS750 front rotors are 280mm
I should have a good "correct" rotor for you. How about $40 to your door?