BYOC Optical Compressor Instructions Manual page 12

If your kit came with a VTL5C4/2(dual element with 5 leads): Orient the optocoupler
so that the printed side is facing up and the "+" symbol on the optocoupler matches the
"+" symbol on the PCB.
What's the difference between the VTL5C4/2 and the VTL5C4? Nothing as far as
this pedal is concerned. They have the same specs, but the VTL5C4/2 has dual elements.
We only need one of the elements for this compressor.
Why bother using the VTL5C4/2 if you only need one of the elements? Economy of
scale. We already had a kit (DOD440 clone) that needed the dual element VTL5C4/2.
Even though the dual element version is more expensive than the single element version,
it was still more cost effective for us to purchase twice as many VTL5C4/2 for both this
kit and the 440 clone kit, rather than half as many of each. We no longer make the 440
clone kit, so now we're using the single element version.