Pretty sure they’re 1/2-13 threadfor the original Eaton axle. My 51 had normal bolts and lock washers, likely original. I suspect that can be confirmed with the parts manual if you want to look.
http://chevy.oldcarmanualproject.com/1929_57chevyparts/index.htm

Newer 14 bolt rear (has a lot of similarities) uses a flanged head bolt with serrations under the flange to resist coming loose. Those could be sourced and used on the older Eaton if you wanted.

Some (other brand/model) axles use studs and cone wedges with nuts and washers. Kind of looks like that’s what you have. What’s the axle from?

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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