This is a special 7/16-20 hex head bolt with left hand thread that we have been supplying the same customer for over 5 years.  It incorporates a special marked stamp per the customer's print and is manufactured using 1018 cold rolled steel, which is carburized to 1700 degrees, soaked for 2 hours, and oil quenched.  Like many of the parts we supply this special part is supplier on an as-released blanket purchase basis.