MasterCard has evolved their logo over the years, but for my money? This Master Charge card was the epitome. I couldn’t find a good photo online so I bought an old one on eBay to document. For giggles I added my name.
I wasn’t old enough for credit in the late seventies, but somehow I remember these referred to as charge-a-plates and plastic.
Bonus link: The Museum of Department Store Credit Cards
Update July 2016: Yay! (Well, maybe not the type)