"Bolt grade strength is determined by the manufacturer. The manufacturer must adhere to guideline provided by ASTM. You will see two types of marks to be on the head of a bolt. The manufacturer's mark is a symbol identifying the manufacturer (or importer). The grade mark is a standardized mark that identifies the material properties that the fastener meets. For example 307A on a bolt head
. indicates that the fastener properties conform to the ASTM A307 Grade A standard. More bolt grade strength information can be found at: http://www.boltdepot.com/fastener-information/Materials-and-Grades/Bolt-Grade-Chart.aspx"
