If it is like the 216, it will be a 2" convex disc. It will probably be the same as the one in the front of the block. Here is what mine looked like:
http://rides.webshots.com/photo/1316417356041873203xTDshC


1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed


My Chevy Master 4 Door is on the Road!