The Navistar 506 blocks are the AMG blocks.

I think you’re getting the 599 (earlier GM, not Navistar) blocks mixed up with the 506 blocks - only the 506 blocks with a date code after 2000 were made by Navistar.

AMG/GEP 506 Navistar Longblock ID: http://www.thedieselpageforums.com/t...ad.php?t=30041