GMP Tools manufactures pentagonal head bolts to secure manhole covers. That’s right, not square, not hex, but pentagonal — another case of security through obscurity. Of course, if you sell bolts with heads that have an odd number of sides you need to supply the corresponding tools to turn them, so they also sell two different sockets: one with a 7/16″ hex drive for impact tools, another with a 19mm hole which you can turn with a rod.
The bolts come in two different sizes. The first measures 3-1/4″ long and has a 5/8″ – 11 thread with a chamfer under the head for countersinking. The second measures 1-7/16″ long and has a 1/2″ – 13 thread with a flat shoulder under the head. GMP tools machines both types of bolt from a “tough grade” of stainless steel.
The 3-1/4″ bolt runs about $25, while the shorter 1-7/16″ bolt runs $15. Remember: to drive these bolts you’ll either need the hex drive impact socket, which runs $40, or the socket with the hole for $14.