casting # ?

[dieseldummy]

A phone conversation this evening got me interested in a block I once had. All I could remember is that it was a 506 casting with small outer bolts and small oil jets. No IH diamond indicating a good casting. I ened up finding a pic I had saved and in it I could read a casting # of 12555506. Does this have any relavence other than indicating the 506 casting? Here is the pic:
http://img.photobucket.com/albums/v622/dieseldummy/61.jpg