I've had a set of MM's between two spider(s) for about 37 years now. They were left outside for a year or two by the previous owner and they were very nasty and corroded. They were just like the gnarly Pantara wheels someone posted but they cleaned up well and have had no issues other than a thin black film that needs periodic polishing to remove. I suspect these wheels must have a small amount of Mg in the Al alloy.
When new these wheels came with a chromate conversion type of coating that offered some degree of corrosion protection. I have one 'spare' (that I am using) that has almost always remained in a spare tire well and the thin conversion coating is intact and exhibits a slightly gold-ish tint.
Sometimes powder coating Al/Mg wheels can result in undesirable side effects (blisters and corrosion) -if the wheel is not properly prepared, coated and cared for.
mag/alum perhaps. i have a set of on my car MMs on my gtv for 28 years. unlike yours mine are unpainted they way they came from the manufacturer. i still shows no signs of corrosion.
i have vintage campys and cromodoras and bbs e50 mg wheels in dry heated storage. they show signs of corrosion on the unpainted faces just sitting indoors. if the MM really are mg its content percentage must be minuscule
never the less 7x14 MM are to die for!