Things that have been retracted for screw locking because they don't really work:
- DIN 127, DIN 128, DIN 6905, DIN 7980: Split and spring rings
-- these are generally considered to aid loosening of fasteners
- DIN 137, DIN 6904: Split and spring washers
-- DIN 137 style washers work quite well for connections on softer materials. For example, for screwing down a PC board to a standoff a DIN 137 washer will probably not come loose. These are/were pretty common for exactly that purpose in industrial electronics.
- DIN 6797, DIN 6798, DIN 6907 Toothed and serrated washers
-- DIN 6797 work to a limited degree on soft materials (e.g. PC boards)
DIN 6796 Belleville washers work for fastening things in general, but would not work in this application since you are applying an external torque. These washers are completely unsuited for that. A toothed/serrated washer will appear to work at first but if you put a bunch of torque cycles on this they'll loosen rather quickly.
So in terms of mechanical locking only locking nuts, and north lock (DIN 25201) or crimping/riveting would work in this application.
That's why I recommended Loctite. It actually works and isn't speciality hardware.