1994 3.5 L. 6G74 Sensor Issue

My engine has 4 sensors mounted in the aluminum water-jacket in front of the Intake Manifold, only reachable of course when the plastic top engine shield is removed. The problem is the Dealer Mitsubishi Parts Manual shows only 3 sensors! And of course, the one which has the top slightly broken on my engine is the one not pictured in their schematic! Has anyone run across this oddity?