R53 thermistor in a glass vacuum tube by ITT These now impossible to obtain items were manufactured during the mid '70's and were used for temperature compensating. a fantastic article by Rod Elliot can be read at : http://sound.westhost.com/articles/sinewave.htm these are probably the last two unused items available.... anywhere ! Status: NOS (New Old Stock) Individually tested components: YES RoHS: probaby not !