Eric, when I looked for/found my no`s , they were right there under the head , next to water pump on pass. side , and were hard to see , just stamped into block , prob. 1/8" ( or less) tall or so , mine was C103858 and YX , if this is any help. This is actually from a 73 motor, which mine was traded to by prev. owner. Also I never saw any no`s like the ones you describe 400GM-5 DN on mine , where were those? Bjorn