Looks like the rt. bank exhaust manifold of 1959-60 389 single exhaust; but I've seen them on 55-56-57 but different. Starting in 61 they brought the left bank tube over and welded it to the rt side.
http://i281.photobucket.com/albums/k...c/DSC02449.jpg
check this out:
http://www.pontiacsafari.com/MPC/Pages/page_0178.jpg