Dual Voltage Processor Model 257t

Model 257 Dual Control Voltage Processor consists
of two identical sections, each of which permits
several applied control voltages to define a single
output voltage according to the equation:

(Va K) + ((Vb (1-M)) + (Vc M)) + Voffset = Vout

The algebraic manipulations possible with this
module include addition, subtraction, scaling,
inversion, and multiplication. Also incorporated is the
capability of using one control voltage (M) to transfer
control from one applied voltage (V_b) to another
(V_c).

https://tiptopaudio.com/buchla/