mislead about late model master cylinders???

SO, was I mislead??
I thought late master cylinders were aluminum....

I just bought one at the parts store for an 85 5th Ave

but it's not aluminum....
also noticed that an 80-ish has a different # for manual brakes....do I need a manual brake master cylinder???or will the 85 power MC be right??

I have big pattern discs on front and 10x2.5 drums in rear

last....I like the Bendix new MC...it has a larger black reservoir than the Brakebest's white one...what do you think?