I'm not sure the exact year Alclad aluminum was used, I believe it started in the early '60s...but you may want to be careful not to sand to aggressively. Try the finest sand paper first, see how it goes and try to feather it out evenly.
If it is Alclad aluminum and you sand through the Alclad layer, the aluminum beneath could oxidize at a different rate and a "stain" of a different sort could appear. As long as it's just surface discoloration, it should come off pretty easily, if the surface is badly pitted, that's where you could run into some problems.
Good luck & post pictures as you go along...