The DOT
http://351c.net/archive/parts/...rev%203-26-18%29.pdf
July 1970 the dot appeared on Closed Chamber 4V heads. for about 1 year of production Ford was casting Closed Chamber Dot heads, until July 1971. that's why you can't use the dot to determine what chamber is hiding inside
but using the chart, you can use the casting date under the valve cover