Bach's E Major Violin Concerto has survived solely in the form of a copy, but in view of its significant compositional substance, and of Bach's arrangement (which has come down to us as an autograph) for harpsichord and orchestra, the work's authenticity is beyond question. The highly expressive violin melody in the second movement speaks for itself: the harpsichord version cannot provide a real replacement for it.
Bach: Concerto for Violin and Orchestra E major BWV 1042