No nut needed as I recall. As you have noticed and Skip confirms the hole in the Firewall is threaded.
Did you buy used, hence the lack of instructions? I will transcribe the IPD info that came with the braces I put on my 242. Most of it will be redundant as it sounds like step 4 is what you're unsure of. But here it is.

1. Locate the threaded hole in the firewall. The passenger side hole is located directly below the wiper spindle and the driver's side is 12-3/4" directly to the right. When in doubt, use the brace to locate the holes by fitting it to the rear strut tower stud and aligning it to the firewall.
The holes may be covered by sound dampening material but the areas should be perforated for easy removal. If it is not perforated, use a knife to cut a hole in the material large enough for the foot of the brace to fit into.
The threaded holes may also have plastic inserts. Remove the plugs.

2. Remove the rear 13mm nuts on the strut mounts.

3. Determine the left and right brace. They are exact opposites with the angled leg fastening to the firewall.
Place the straighter end of the brace over the rear most stud and locate the other more angled end onto the firewall.

4. Put the bolt through the aluminum spacer and then through the brace and screw into the firewall.

5. Replace the nut onto the strut mount and fasten securely. Do the same on the other side.

6. Check all fasteners for tightness and you are finished.

Note: Use a drill to elongate the holes in the strut braces for cars out of normal alignment specs.

Reads sort of like a translation from another language. They don't give a torque value for the firewall fastener. I used the value for the strut top mount nut, 15 lbf-ft. I think the final note is a little silly. To my way of thinking, if the car is out of normal alignment, get it properly aligned.
Someone recently mentioned a clicking noise from the firewall fastener when cornering. I have never experienced this.
You might also want to consider the lower tie bars if you don't already have them.
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'80 DL 2 door, '89 DL Wagon