The PAiA 4710 Balanced Modulator can function as a 4 quadrant multiplier with a sum and difference output or as a 2 quadrant multiplier (e.g. VCA). The specifications below say the controls are convenient for a VCA or a sine wave doubler. The banana jacks provide the control input for a VCA but there is nothing to conveniently provide for a frequency doubler. Normalling the audio to the carrier jack would accomplish this but is not documented in the manual. This kit was $22.50 in the 1976 catalog.
PAiA 4710 Balanced Modulator User Manual
This particular build used a can style MC1496 IC.
This image shows the PCB traces.
This scope image shows a frequency doubler. I used two different frequency sources but tuned them to the same frequency.
This scope image shows a sum and difference with two unequal inputs.