I have a pair of 690 heads. 1has pinned rocker studs and valve guide bosses around 1" diameter with casting number CFD. and 68 with GM1 between them. The second has no pinned rocker studs and valve guide bosses around 9/16 diameter. The number is G2581. My question is dors anyone what years they are and when and where did they not pin the studs? Is this small diameter boss stock or has this been modified? These both have a mild porting job done. Any info would be appreciated.