Back-up or engineering prototype for a solar X-ray spectrometer flown on the stabilized sail portion of the OSO-2 satellite. The instrument was designed to map solar X-ray sources, to measure emission bursts and to measure emissions from prominences above the solar limb. It comprises an array of four Geiger counters fed from a common gas source. The three burst detectors were aimed at the solar disk and a background detector was aimed away from the sun. The satellite, placed in orbit on a Delta launch vehicle on 3 February 1965, was the second of a series of eight Orbiting Solar Observatories. OSO-2 returned solar X-ray, UV and gamma-ray data for nearly nine months. The artifact is part of a suite of NRL-designed instruments transferred to NASM by the Laboratory in December 1983 (see Catalogues #s 19840019 through 19840025000).