GT40 irons came on 93-95 Cobras; 95-first 2 months of 97 production Explorer/Moutaineers.
GT40P irons came on last 10 months of 97 - 2001 Explorers and Mountaineers
GT40Y's, GT40X's and GT40Z's are all aluminum and available from FRPP. Y's flow like the P's more or less; X's a bit more aggresive; Z's most aggresive of all.