That instrument is a trombonium, ca. 1960-70. I found one just like that when I took a job at an elementary school. The school was over 50 years old, and some of the intstruments were original school materials. I found a trombonium, and for the life of me could not figure out what the heck it was.

If you google "Trombonium" you should be able to find an exact picture of your instrument. Mine was Manufactured by the H.N. White Co. and the serial numbers were stamped on the bottom of the first valve (the 1 & 3 valve were also labeled)