The grade marking used today did not really come into existence as an industry "standard" until about 1969.
FoMoCo began to require fastener vendors to follow a common grade system with the various radial lines on the head.
Prior to 1969, FoMoCo vendors were only required to put a vendor I.D. mark on their fasteners supplied to Ford.
The new 1969 FoMoCo spec sheet on fastener grade markings included the common marks still in use today and also shows a grade 6 with four radial lines.
The O.P.'s L600 engine was made from 1950 and 1989. Plus the bolt head he shows with the CP on it fits the bolt head logo for Chandler Products in Cleveland, OH.
They are still in business today and there may be counterfeits coming in from overseas. But I doubt it is a counterfeit from 1950-1989.
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