I can not find this part number to help me identify what year the motor is....The rear of the block has:

12551364

The crank has:

12552216

TO2 along the lower pan rail on the Drivers side of the block
SO2 along the lower pan rail of the Passengers side of the block

12551364 stamped on the rear cover

Also found 608 or 809 depending on which way you read it upside down or right side up....it appears to be 608 however.

Found 6.0L stamped on the front and rear of the block and aslo GM-SMCO

No heads and the bottom end appears to have NO MILES on it, NO CARBON on the tops of pistons, crank journals, main caps, rods all look to be brand new OR rebuilt. Is this an 05 block becauase my catalog only goes up to '04 part #'s. OR????????????????

 

Check out Automotive Builder.

The casting number from the block and crank looks like it could be from 01-04 by the website given.

 

I have the "How to Build High Performance CHevy LS1/LS6" and for the 01-04 it has the part number of :

12551365

On my block it has the :

12551364

That is what is throwing me off. Is my book wrong do you think???