5/16-18" heat treated twelve point bolt. These bolts are made from heat treated steel and are grade 8 specs. They are black oxide coated and are only designed to work with RCD's PSI screw blower mini snout blower belt guard.
Bolt uses a .375" wrench head and is 5.00" under the head with 4.00" of shoulder and a 1.00" of coarse thread.
Third picture shows the belt guard that the bolt is designed to be used with.