They're called "Palnuts" and the best place for 'em is the trash can. They're totally unnecessary- - - -and likely to work themselves loose and end up in the bottom of the oil pan. They're about as useless as the cotter pins and castle nuts used on the old International truck connecting rods. A properly-torqued rod nut just doesn't come loose, and one that's not torqued correctly can be safety wired, cotter-pinned, or double-nutted and it's still going to get loose and wreck the engine!
Jerry
Edit: If you think the rod nuts need a little help, just clean the threads thoroughly with a solvent like brake cleaner or lacquer thinner, and add a drop of blue LocTite before torquing the nut. It will have MUCH more holding power than a palnut, and if the stuff existed 60 years ago, that's probably what Chevy would have used.
Jerry
