When Magnum talks about his high school football coach, "Grant," in one scene he is looking through his yearbook with the cover clearly shown as "Shield." The yearbook was from Selleck's actual high school, US Grant High, Van Nuys CA 1962, his Senior year (although the date and school name are not visible). The page he turns to with the shot of the Varsity football team was actually his school's team although Selleck played Varsity basketball and Varsity and JV baseball according to the information provided under his class photo.
The term "feet of clay" refers to a fundamental character flaw in a person who is greatly revered. It is an ancient reference to an otherwise magnificent marble statue that stands on feet that are weak and easily damaged, causing the statue to fall.