I have a NOS COSIM Variometer for sailplane, gliders and it was manufactured by Cobb-Slater Instrument CO., LTD., Matlock, England in the 1940 and 1960. It's a pellet type of indicator in a bakelite brown enclosure, it measuring unit is ft/sec, a red pellet for downward and a green pellet for upward movement. It would be appreciated if someone knows what this variometer is worth. Thank you