There were 2 different castings, I found out the hard way buying a used set from a member here many years ago (circa 2010). And they are just different enough that if you set one next to the other you can see the ports are slightly different. I confirmed by cc'ing the chambers and one head was about 3cc less than the other across the board. IIRC one of them was used only on 5.9L engines in the early-mid 90s then they switched to just using the same heads on all engines. The 5.9L ones were cast at a different foundry and were the ones with slightly bigger ports and chambers.