Axle is the internal torque transfer , that is why the anti Rotation stop is required.
derailleur bikes don't have any torque transferred to the axle, Planetary internal gears do.
Parts of the internals . rotate backwards , then forwards.
Do you bother with anti rotation washers?.. not exactly in plentiful supply.
1 set per hub sold, for over 100 years, is plentiful, to my thinking..
Edit; if the axle rotates, one nut will get tighter, the other one will get looser.
The hub was sold incomplete? guess that was the trade off, missing parts
as you said:
the dual drive came up at the right price
S-A axles are flattened, the anti rotation washer slips over that and adds to the
un round ness of it.
I think, its the .75 gear that gets the torque resistance ..
FWIW, Mountain drive crank has a torque stop, lever . [Schlumpf]
Brompton version its lever has a button sitting on top of the piece behind the BB shell
that the rear portion's hinge is on the back end of.. torque is CCW, pedal turns CW.
the speed drive, just, relies on the friction of the chamfered BB edges
mounting rings, being pulled into as its tightened into the frame.