Seek and you shall find..

The Letters mean "A" Magnesium/ Aluminum alloy "S" is for silicon. 4 is 4% aluminum and 1 is for 1% silicon.
Both alloys are still available and have working temperature ratings of up to 300F degrees for AS-21 and about 290F for AS-41.
AS-41 is a stronger alloy but AS-21 has more ductility and more heat conductivity. You would have to keep the 300F for at least an hour for any detrimental effect to take place.
Neither alloy is normally heat treated. Both cary F designations to be used as fabricated.
There are many Magnesium alloys with temperature ratings exceeding 600F and up to twice the compressive and tensil strength.