the xxx blocks where cast in the USA where as the yyy blocks where cast in Mexico and have a higher nickle content which make them stronger and are popular for forced induction and heavy nitrous applications.
Someone told me that the XXX or YYY castings in the blocks were a place that was prone to cracking or were thin casting in that area. the XXX or YYY was put there to aid in the rigitiititittiiittttyyyyyywhywhywhy (it's 2am, i am tired) of the casting. But i could be wrong. Just a thought to add...\
Its in the valley, sorry for the tiny pic.
This is an Explorer block, its the XXX... my original '91 block is a YYY.
I dont think there's any difference in the XXX; YYY blocks I wasnt concerned about it when I swapped them. It probably denotes a different casting plant, or casting machine.