Could be. Odd that they're stuck off all alone the way they are.Maybe I'm completely wrong on this, but I'm not sure I can see any polarity markings on D4 or D6? Are we sure they aren't surge protection diodes of some kind?
Good catch, pfofit.What makes you think they are Zeners. Elsewhere on the board, red circle, there is a zener clearly marked as zd1.