To the left you see two different styles of  lug nuts.  The conventional acorn lug nut is shown on the left.  It is a 13/16 hex, 60 degree, 1/2 - 20 thread, open end lug nut.  Typically, you see this or the 3/4 hex version on cars with conventional lug nuts.  

However, its not what I wanted for my Cobra R rims.

The nut/style on the right is known as the "bulge" acorn lug nut. The 60 degree cone seating area is much bigger (one could say it "bulges"..).  This is because it extends outward and is continuous around the circumference unlike the conventional lug. Otherwise, it is still a 1/2 - 20, open ended 13/16 hex lug nut.

In my opinion, the "bulge" gives the nut a better clamping area on the wheel.

I ended up purchasing them from the Gardner-Wescott company at 1.800.521.9805.  Their part number L-1304-1, 1/2 - 20 13/16 hex open acorn bulge lug nut. $0.71 cents each.