^^ How about "M"'s I have seen CE2M12345 before
Need to know more - at least casting number and casting date.
I have not seem M used.
Options are
Complete assembly, partial block, fitted block, bare block.
Complete assembly is a long block.
A partial block is basically a short block - block, crankshaft, camshaft, timing gears, pistons and connecting rods.
A fitted block includes the block, pistons, wrist pins, rings and bearings. I am not sure if the rods are included or not.
For this case, the SO might be the Service Order number put there by the dealer that did the warranty work. I am not sure.