Transmission arms

i'm not sure what's *correct* but i've seen thin flat washer + lock washer and nut, flanged nut with serrations as well as thin flat washer with a "stover" type lock nut.

so some type of "locking" hardware should be present.

if you don't care about originality, a nylock could be a viable alternative. with any hardware i'd want some type of flat beneath the fastener-- so either a flange nut or a flat washer.