This doesn`t really answer any question but I took a new axle, rim and speedometer drive and checked some dimensions. This is of course a 16" Oscam rim with an according speedometer drive.
W/o the washer on the right the axle protrudes about 2mm more than the fork leg is wide. The thread is only 20mm long and protruding part w/o thread is ca 34mm which means that the hole in the left fork leg should be more than 34mm deep before the thread begins.
If it was less the end of the thread on the axle would touch the slider tube meaning there would be axial play and need of that washer, right?

G.

G, Thanks for the investigation.

The axle on my 1991 907 is the same dimensions as the one you show. Without the washer, and all assembled, the axle protrudes both legs about 1mm, which seems about right. If I thread in just the axle into fork leg, I can get about 8mm of thread showing on the other side of the fork leg, so I know I'm not bottoming out the step on the axle.

The big difference on the 907 is the left fork leg is only 45mm wide, compared to the 50mm for the 750 and 906 fork legs. The 5mm difference is on the inside of the fork leg, and allows the use of the newer wheel and speedo drive(wider). This (I think) is biggest challenge to putting a 17" wheel on M1R forks.

Still haven't dreamt up an engineering point of view that can explain why someone decided they needed that washer. The axle and the center bore of the bearing are certainly hard enough steel to take the load.

Cheers, Phil

Paso750:
I see your point. Do you think maybe Ducati put the washer on the right so the shoulder of the axle would distribute the pressure on the the bearing better?

I don`t think so. The thicker part of the axle is 22mm which means that the step is 2.5mm high (17mm axle). The inner race of the bearing is about the same height.

Phil,

this whole story made me curious. I didn`t want to remove the wheel so I just pulled out the axle far enough to measure the fork leg.
I measure a width of 53mm. As I had posted last time the axle protrudes 54mm on the left side of the rim.
The thread in the fork is 20mm wide just like the one on the axle which isn`t really relevant.
More important is that the step where the diameter is reduced to the thread is about 23mm wide which means that the larger diameter unthreaded part is 30mm wide.
The unthreaded part of the axle measures 34mm (last post). Based on these numbers the consequence is that even if the axle was screwed in until it bottoms out there`s still a gap of 4mm.
It must be less than that as the washer is 3mm and the shim between speedometer drive and fork is 1mm (the 907ie doesn`t have that) but I`ll give that to my not very precise measuring.
However, if I haven`t made any thinking mistake this for me proves that on the P750 & 906 you need that washer or the wheel won`t sit tight.

G.