Nobody doubts that the C's were 406 heads and that they were good heads,I just don't buy into your statement that B's weren't 406 which is exactly what your first post stated.Not only were the C2SE-B's and C2SE-C's used on 406's so were later C3AE-C's,and while some of the earlier heads may have been equipped with the spring seat cups if they were meant for a racing application,as I understand it the C3AE-C's were the ones that introduced the larger exhaust and spring seat cups on the normal production line.I have examples of all 3 and they support that,the C2-B's and C's weren't machined or equipped with the cups or larger ex. valve and the C3-C's are.As far as I'm concerned at least in this instance the Mustangtek information is pretty accurate.