As a Harvard University student, Leonard Bernstein wrote two movements for strings in 1936, and asked the New England String Quartet to test them in rehearsal. The 18-year-old gave his manuscript to the ensemble’s leader who stowed it in a drawer. Apart from occasional outings for domestic performance, the score remained buried until it was put up for sale decades later. The impressive first movement, the longest and strongest of the pair, received its first public outing in 2021, performed by the artists on this fascinating world premiere recording. “Movement II” surfaced soon after in the Library of Congress and stands here as brooding companion to the jazz-infused syncopations of “Movement I.” Elegies for Violin and Viola (1932), a somber work by Bernstein’s friend and early mentor Aaron Copland, is an ideal coupling.