Probably this: http://en.wikipedia.org/wiki/IBM_Selectric_typewriter http://en.wikipedia.org/wiki/IBM_Selectric_typewriter The golf-ball typing element has a l

Probably this: http://en.wikipedia.org/wiki/IBM_Selectric_typewriter http://en.wikipedia.org/wiki/IBM_Selectric_typewriter The golf-ball typing element has a lever at the top, which opens to unlock the ball in order to change fonts. Besides the carbon ribbon, there's also a small white-out ribbon, to correct mistakes.