Engine What's this bolt?

While searching for grounding bolts on an '89 5.0, I found a 1-1/2 to 2 inch long black bolt on the front driver's side of the engine. It's in the vicinity of the oil filter & oil pressure port. It goes into a hole that's larger then the bolt and threads in deeper in the hole. Obviously not good for a ground, but what is it? If I loosen it slightly and run the engine, no oil comes out.

Also, any suggestions for engine block grounding bolts? This is in a Ranger and the rear of the engine is inaccessible.

It's not a timing cover bolt, it doesn't attach anything to the engine. It's on the front of the engine, just above the front driver's side corner of the oil pan. It goes into a raised stub and engages deeper in. There's another bolt with a similar head size just inboard but I can't get at it. A photo from above is impossible but I have the front end raised on jacks so I may be able to get a picture from beneath. I'm not sure how to post a photo.

I found the sensor ground (I think) on the top front of the engine. It's a bolt with a stud coming out of its head. It's hard to explain but it's obviously meant to be bolted in and then a nut (and lug) go on the protruding stud. I added a #6 wire to this and ran it to the passenger side frame rail, my ground "buss bar" which has the battery negative and the starter ground connected to it. I wanted a larger wire to accommodate the alternator ground but that's the largest that would fit on a 5/16" lug (well, a #4 would fit but Home Despot was out of it). This wire may be redundant because the engine is still -.53 volts relative to the body (fenders) but it can't hurt. Interestingly, this bolt also goes into a larger hole and engages deeper in. My next step will be to install a #14 wire from the rail to the body. I'm not sure whether the ECU uses the body at all since there is a ground wire going to the engine, but my EGO's have been constantly reading lean lately and this definitely affected my oil pressure gauge. So the voltage supplied to the gauge is definitely referenced to the body. My opinion is that I should see zero volts between any two pieces of metal on the vehicle when the key is in "run".

This all started when my electric oil gauge kept reading higher and higher. I replaced the sender (same result) and then ran a braided steel line from the engine port to a couple of tees at the driver's fender. The tees have the original electric sender, a mechanical oil gauge that lives under the hood (told me the pressure is fine), and an idiot light sender; I'm covered. I soldered short wire with a spade lug to the tee with the two senders and grounded that to the fender a couple of inches away. The electric gauge then read dead center, so I knew I had a ground problem and began poking around with a meter. Who knows what else this is affecting?

In the process, I found that the gauge is a true gauge but Ford put in a switch sender that closes to ground at 6 psi and above. The idiot light sender closes to ground at 6 psi and below. I see there's a 20 ohm resistor in series with the gauge to make it center. On the gauge input, zero volts will center, +5.5 volts (unconnected) will read zero psi , and -1.5 volts will read max high. Obviously, there will never be a negative voltage in the system but that's what's needed to make enough current flow with the 20 ohm resistor in to make the gauge read high. When I get time, I'll put another resistor in series and measure the voltage on that. That would tell me the resistance of the gauge so I'd know what kind of sender I need to make a true gauge by shorting out the 20 ohm resistor. I think I've seen a thread about doing this but no one mentioned what sender to use or where to get it. Does anyone know?

While poking around I found that my driver's side motor mount bolt was loose to the point of 1/2" of movement. The nut came off with only two turns. Whew. I replaced it with a grade 8 bolt and a locknut.

So I won't be using the unknown bolt but it's black, which seems special, so I'd still like to know what it's for.

Here is a photo of the bolt from underneath. It's the one more to the right. The corner of the oil pan is below it and that's a water hose in the upper left. actually, I'd like to know what both bolts are.
The tube with a blue "E" to the right is the oil pressure port.

After using another bolt to ground the engine to the passenger frame rail and also grounding the fender wall to the rail my engine is now -63mV (-.063 volts) relative to the body. That's much better than the -.53 volts than before.