thanks to Mike West
From the ASM Metals Handbook, V.2, 10th edition, 1990
Room Temperature properties (Al=Aluminum, Mn=Magnesium, Si=Silicon):
"AS41A has creep strength much superior to that of AZ91D or AM60B alloys at temperatures up to 175C (350F)... AS21 alloy exhibits even better creep strength than AS41A. However, AS21 has lower room-temperature tensile and yield strengths, and it is somewhat more difficult to cast."
The newer cases are made of AS41A which is better than the original AS41 material and even the AS21 which was used for awhile.