I don't think its a lock nut. At least in 1956 they weren't I seem to remember a washer with groves for locking under the nut (?) but its been many years. Anyway, I had to make one for an old Dodge 1/2 ton. I found a nut of the proper thread configuration and had it split to the right thickness (on an industrial bandsaw *only because they had one where I worked that could slice a .06" wafer from a piece only an inch long*)

A carefully wielded hacksaw and a vise would work too in a pinch

Scott