D15d-replacing solenoids and rewiring -need help

The D15 diesel I recently acquired is a 12v, single battery, negative ground with alternator set-up.

It has 3 solenoids on it. It should only have 2 - judging from the shop manual.

I am going to refer to the 3rd solenoid as the "Mystery Solenoid" For I have no idea why it's on there.....I did remove as couple of broken switches that were mounted on the tractor here and there with no wires going to them. I presume it may have had something to do with those once upon a time.

The left terminal on the manifold heater solenoid got cherry red and melted the wires off of it this morning when I tried to start it. I presume the sketchy wiring and rust are to blame. Since I need to replace wires anyway and replace this solenoid, I want to eliminate the "mystery solenoid" in the process; So, I was hoping the electrical wizards on here could tell me where the wires on the mystery solenoid should go or if anything else is wired wrong based on my description and pictures:

First. lets start with the solenoid on the starter itself.....the only thing I see a miss there is it has a metal tab hanging off of it, just dangling there. Should'nt the one end of the tab be placed on the starter terminal itself? Why would they have removed it in the first place? Here's a pic:

Now for the Manifold Heater Solenoid:

 - It has a wire coming out of its right terminal going to the manifold heater. (as it should be accrording to shop manual)

 -It's left terminal that fried had two big wires; one running up to the positive terminal on the battery and the other connecting to the bottom terminal on the mystery solenoid.

-It has two tiny wires on the top terminal. A red one running back to the heater button (As it should be) 

- and a green one running back into the wire harness and I cant trace it from there. I presume it goes to the key or amp gauge.

Now for the Mystery Solenoid:

-Has a green wire on bottom terminal that goes back to the bottom terminal on the starter solenoid.

-Top terminal has a heavy wire which connects on to the starter terminal itself.

-Small terminal, on side, has a small black wire which goes up to the key switch.

Well, there you go, hope someone can help me out.

Also, when I look up a new manifold heater solenoid to order - The OEM # keeps coming up with 6v saddle mount solenoids.

I did come across this one at jensales. It is a flat mount, and claims it will work with either a 6v or 12v system. I'm pretty sure it's the one I need: