Aluminium 215 connecting rod nuts (see pic)

I am rebuilding the engine for my 63 Skylark convertible. It's a 215 Aluminum. Everything is off except for the rods, pistons and crank. The rods have a weird looking nut holding them on (see pic) Are these normal and what are they called?? Where can I pick up a socket to get them off?? Thanks
Jim

 

That is an aftermarket 12pt nut. A typical 12pt socket should work on it.