A Fan, Here is a link that should help.
http://www.fordgarage.com/pages/diamondblock.htm
The later castings are desirable for the guys wanting to hop up a banger as the castings had better material and structure. The number on the block is probably not the number for the block itself but was transferred from the previous motor that was removed?
Hope this helps. I know others will chime in.
Good Luck