Piston, Walter: Symphony No. 6. With Martinu, Bohuslav: "Fantasies Symphoniques" (Symphony No. 6). Boston Symphony Orchestra, Charles Munch. RCA GOLD AGL1-3794 -- Gold Seal has been RCA's treasure trove for forgotten older performances that could still find a market if not priced as high as top-of- the-line modern day offerings. This reissue of the world-premier recordings of these two works was commissioned by the BSO for its 75th anniversary. (I doubt the record companies are showing as much interest in the BSO's 100th, going on this year and next in Boston: How times have changed!) Both are neglected works -- the Piston a fine, rigorous, solid work; Martinu's an inventive, unusual, engaging, and fascinating creation. The masters were made in stereo, though we hear them that way for the first time. The performances ar e vintage Munch -- vital, electrifying.