Jim,
Is the one you got from Cummins cupped or wavy so that when the nut is tightened it maintains a constant pressure between nut and gear? Something like that probably would be better than the flat one. I sure wouldn't want to use a common split lock washer.
I have read on this forum about the difficulty some of the folks are having with the gear slipping following a timing change.
Consequently, I used almost a full can of brake cleaner and blew it out with a mild blast of air before replacing the washer and nut.
I torqued it to 150 foot pounds and it has held fine.
I rechecked it after about 3000 miles just to make sure.
Do the pumps for the higher horsepower engines require more torque to turn them than the pump in my lowly 160 hp 94?
If so, that may have prompted them to change the washer.
Howard