To expand a bit on that - if you look at the left-hand board, it has two rows of holes for the pot to be soldered two, but the pot used is a single pot (compare with the two pots on the other board which are both double pots). If you look at the left hand board between the pots, you can see
and the bottom line says that if you have a piezo, you change the single variable resistor to a dual one. That also tells us the value of the blend pot.
Whether such a pot exists is another matter. I think it's unlikely.