There's about a page on it in my monograph on the HSL published by Steve Ginter. It used the HSL dynamics (rotors, transmissions, drive shafts) with a new fuselage and three turbine engines, T58s or T53s. It was similar in size to the Boeing Vertol 107 that became the CH-46. It was first proposed to the Army and then to New York Airways as a 21 to 24-passenger airliner. The Army evaluated the BV 107 and contracted for a bigger version, the CH-47. New York Airways bought a civil certified version of the H-21.