Enhanced grip and security, spline-drive socket sold separately. Lug nuts are sold EACH.
The antique connotation of 'Coker' as source, along with it being a 'clearance' item, adds up yours maybe being some old discontinued pattern. Maybe call theirs and other suppliers' techs and see it you get a hit on the exact origin
It seems like those old strange-shaped sockets they used to sell with rounded down edges to grip deeper into the meet of rounded over hexheads might fit???? Otherwise just some heat and chisel time just nudge them loose.