Nissan: 1993 Nissan Quest - Oxygen Sensor Question

I've been told that Nissan Quests has three oxygen sensors by a local
auto shop. Since the downstream O2 sensor was not added until the 1995
models, I can only imagine there being at most 2 on our 1993 model. I
see the location of the upstream O2 sensor. Where is the other?

-OB