Wouldn't a 1938 1197 Packard be a 120 convertible coupe? Shouldn't a 1938 120 Convertible sedan have number 1199?

I use the word 'kindly' thanks to a teacher from almost 60 years ago. I would request their phone number and speak directly with the individual. If you offer to pay with a check, you have their name and address. If they balk at any of that, then you know that the deal is suspect.

It was quite a struggle, but I was able to remove the horn button on a 1938 banjo steering wheel by working a thin wire under the button. It is
hard to get a good grip on the button but the wire allowed me to work it off with careful patience. After getting the wire under the button, I
worked the ends of the wire around the edge at around 1/3rd of the circumference. Perhaps moving the wire around the underside of the whole button
helped to work it loose. Yes, you move it counter-clockwise. Good luck

Bob was at the National meet in Brookfield in June with a LOT of early '50's Packard Parts. If you need anything for those cars, contact him!

I've never seen those on a Packard six. First, gently remove the hubcap by working a small screwdriver or putty knife around the edge.
A cloth placed at the fulcrum would be wise. Once the hubcap is removed, you can easily remove the thin 'donut' without damage.

I have a 73 page Owners manual dated August 1923. The top of the cover reads: INFORMATION SINGLE-SIX Models 126-133. The first page is dog-eared and page 3 looks like a child cut out the Packard logo from the middle of the page. I also have a 1941 Packard Owners manual for the One-Ten Special, One-Ten Deluxe and the One-Twenty.

There is a local quick oil change shop near me that accepts used antifreeze. Try checking with one in your area.

I want to rebuild the master cylinder on my 1936 120 without damaging the knurled edge of the plug. Years ago, I would simply use a pipe wrench when rebuilding
a master cylinder in an old car but now I want to avoid damaging the knurling. Was there a special wrench made for this and if so, where would I find one? Could
I wrap the part in copper sheet to avoid damage? Any suggestions would be appreciated.

I think that you are on the right track but the closed end goes toward the windshield. Good luck

1936 120 open cars did not have vent windows. That changed for the 1937 Juniors.