What is that sensor that screws in toward the bottom driver's side of the block?

 

Tom,

If it's near the oil filter adapter, it could be the oil pressure sensor. If it's closer to the center of the engine, it could be the detonation (knock) sensor, but those are usually on the right side, not left. What color wires are on the connector?

 

It screws into the block, right? Not the exhaust manifold? If it screws into the block, it is the temp sensor for the temp gauge, while the ECM's sensor is atop the intake manifold

 

Here is a large picture.. Sorry, I did it to retaint the clarity. 98K this will show you all sensor locations and what they are. Right out of the shop manual.
-edit-
Here is some other information I have, sone of which is not in the picture, and may answer your question.
#1 Mounted on top of the Intake, front pass. side, is a single blade unit.
Is this for the electric fan?
Not sure about #1

#2 Mounted on top of the thermo housing are two units, one has a ½" pipe thread, hex base
with a plastic tower about 1 ¼" tall. It appears to have had two 1/8" plastic nipples,
which are broken. Nothing hooked to it when I got the car.
#2 is some thing for the evap canister (which I broke off my thermo housing)

#3 The other one (re #2 above) has a 3/8" pipe thread, brass hex base. Some damage to the
plastic here as well, but the cap is oval shaped with a 1/16" pin and green wire with
a white stripe.
#3 is the ECM temp sensor

#4 Cyl Head mounted unit on drivers side between #1 and #3. I’m almost certain that it’s
a sending unit for the Temp Gage?
#4 is for the dash gauge temp

#5 Cyl Block mounted, mid engine near pan rail. I’m quite sure that would be the Knock Sensor?
#5 knock sensor

#6 Cyl Head mounted unit on Pass. side between #6 and #8. Hex head with 3/8" pipe thread, small
hole thru the middle for round pin connection (no connection when I got the car)?
#6 upper is the thermo switch for the elec fan on a F.I. car

#7 Block mounted just to the drivers side of the distributor. There are two units ‘T’eed off
from one ¼" pipe thread hole. I’m sure they are oil pressure related. One for the oil pressure
guage and one (I believe) cuts-off the tank mounted fuel pump in the case of no/little oil
pressure. Which is which? The larger unit is Bell shaped with a single blade connector and the
other looks like a typical sender having a two-blade connector. Both units have the ¼" PT.
#7 the bell shaped one is the gauge sensor