The exhaust manifold with a casting number of 406301 and large letter X is marked with question marks in the wiki site. I have two 1971 455's 4bbl single exhaust engines, one from an Olds 98 and the other a Delta 88 and they both have this manifold on the drivers side. It has an open plenum like the small block #1 manifold but the log portion is much larger. Both engines also have the letter T casting 398708 on the passenger side. John