High quality silkscreened aluminum panel. Made in Germany.
 
From the original catalogue:
"Model 158 Dual Sine-Sawtooth Oscillator
Two independent oscillators in one unit. Frequencies are continuously variable from 5 cps to 20 kc and may be controlled internally or with externally applied control voltages. Waveshape is continuously adjustable from sine to sawtooth; oscillators may be wideband frequency modulated."