The grade designation comes from the automotive industry, their specification SAE-J429. There are ASTM standard bolting specifications that have compairable bolts. The problem with the two standards is that the marking requirements are different. An SAE Grade 2 for example has no markings, it also has two different strenght levels depanding upon the diameter of the the bolt. For 1/4 through 3/4 the bolt has a 74,000 psi tensile strength as a minimum, over 3/4 through 1 1/2 the tensile strength drops to 60,000 psi. The ASTM A307 is a compairable bolt (with no markings) but its minimum tensile strength is 60,000 psi no matter what the diameter. This is exactly like MarkJC stated, you don't know what you are getting and with a minimum tensile strength specification you can end up with bolts of the same grade with drastically different mechanical properties. I am going to scan a bolt head identification chart and will try and post it tomorrow.
Randy