I'm the very fortunate and proud owner of a 2005 M1123 HMMWV. I understand AMG installed GEP engines in them after 2001. The 6.5 N/A engine in mine has a casting# that ends in 506, but does not have the number 506 cast in the lifter valley. My engine has a tag on the back that says it is a GM rebuilt engine that was done in 2010. The date code (I think) in the back of the block reads: K170. I was told that the US military had a lot of GM produced and rebuilt by GM engines that were lying around and used to replace engines in HUMVEES that needed them after 2001. My question is: what year is my engine and why, if it is a GM engine, does it have a casting number ending in 506?