What does SGK mean below E Pluribus Unum on a 1976 50 cents piece?

It's actually SGH, the initials of Seth G. Huntington who designed the reverse of the coin.

All U.S. coins currently carry their designers' initials. Some are very well hidden (GG for Glenna Goodacre on the Sacagawea dollar, for example) and others are quite visible (JS for John Sinnock on the dime)