I have never seen the fiber washer, but what I have seen on many gearboxes is sign that the bolt have been loose and wallowed up the hole and/or cracked the lug on the gearbox. I can only suspect a broken fiber gasket beeing part of the fault there, loosing up the clamping force.
I would personally not use a fiber gasket anywhere in the joint there, only a steel or aluminium washer on top to reduce wear from the bolt head on the gearbox lug and a good lockwasher under. And I always use without exception -spare a few places- LOCKTITE on all threads regardless if I use a lockwasher or locknut!
A tip is to weld a wing on the bolt head that prevent the bolt to spin at tightening and removing, that will spare you from fiddling with two spanners at the same time in that cramped space and awkward position.
The lug hole on the fork for the plunger cylinder has to be square to the front axle line or the risk is that it will bend on the thin cylinder stud when you brake and the fork is bouncing up and down.
That single feeble little cylinder stud is the only thing that prevents from a destructive, dangerous and possible fatal disaster. Totally underdimensioned in my opinion and a subject to dangerous wear, so a thorough careful inspection and fitting is imperative and as new cylinders are available reproduced, fitting a new one is very high on the list of most desired safety improvements on the bike.
The measure for the fork bracket is not on the fraction as the cylinder and plunger is long, but the measure I have is 190mm (7,5 inch) fork bell crank hole center/bracket hole center.
