I am assuming the bolts you have, have holes in the threaded shank to put a cotter pin thru the slots, and the hole, after nut is tight.
I don't think locking nuts with "locking features" were used in 1949. The cotter IS the safety/locking feature. The only "integral locking feature" possible back then could have been "deformed threads type". In the case of castle nuts, that may mean the "castles" were bent inward. I doubt you have that. If you do, it could be a PO added nut. Which I say is a wrong nut. That kind can damage the bolt threads if the nut is harder than the bolt.
Castle nut is a word for castellated nut. Comes form the castellations on a parapet wall of a castle.
You could also have a slotted nut. Either will work.
I respectfully assert that your analysis of what you saw is mistaken. The holes are there for a cotter pin as you rightfully say, which is used with castellated nuts to lock them. In this case, you torque to minimum or mid spec, then tighten to catch a cotter pin hole. 60 degrees max rotation.
This case has nothing to do with the other use of castellated or slotted nuts and cotter pins on a spindle/wheel bearing where, in many cases, you "back off" to a hole to crate proper end play/bearing pre-load. (depending on tapered roller or ball)
