I built a Jürgen Haible Frequency Shifter using a printed FrontPanelExpress panel. This is a narrow format Synton panel to fit the available rack space. It is very tight. The SUM or DIFF is selected by the FDBK switch to the FDBK attenuator to a second input on the module.
I sandwiched the PCBs together in the center of the panel to have access on each side to wire to the panel. The bracket had to be flush with the right edge to allow clearance for the banana nuts and potentiometer bodies. The wires soldered to the PCB are for the 0° and 90° triangle outputs.
I had to solder wires to nearly all of the panel components and then mount the PCBs as they limited access.
Here is the sum and difference outputs from a 200 Hz external input and the internal oscillator at 300 Hz.