There are two types of plugs used, it depends on the year of your engine block, the depth of the "step" where this cam plug goes , on the early block is .225 inch deep from the outer most edge (that would use the "concave plug") and the depth is .385 inch for the later block (that would use the "double step" style plug). Early block is about pre 74, later would be about 74 on. Pic below is 74 on.
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