This girl is Midge Hadley, the best friend Barbie ever had and was first introduced in 1963. She was made from the same bodymold and with the introduction of a friend the stamp changed on the SL-body. It's marked on the butt:

Midge T.M.
© 1962
Barbie^©
© 1958
by
Mattel, Inc.

Midge came dressed in one of three two-piece bathingsuits, depending on which color of hair she had. The blonde had a two-tone blue bikini, the titian's was orange/lime and the brunette wore a pink/red swimsuit.
The dollpic to the left is how I bought her, dressed in that knitted blue dress.