I have all the parts to check this out, I'll try to get to it later today.
72 lowers are aluminum. MikesXS reports the calipers don't fit either but I think that is just a minor "change a mounting stud" deal.
I looked through the fiches too. The thing is you can't trust the 73 fiches very much, even the ones on the Yamaha site have some errors. Yes the 72 axle # is superceded to a newer number that is "probably" the universal XS650 front axle but again I don't completely trust Yamaha on this one, hold breath, stay tuned to this channel.
What is indisputable is that Yamaha changed this design after one year for good reason, among other issuess the parts count was very high on the 72.
If the bike may be a restoration someday having the "correct" brake will matter.
The thing is I don't think the issues are very big, make those rubber pads, possibly replace the bearing (standard parts house bearing is my guess) and set it up properly and it's good to go.