For me if you jack up the car on that side with the tire still on, grab the tire and try to rock it side to side. Also in and out from top to bottom. That will help show any play in the front suspension parts. It it rock in and out, that is usually kingpins. It it rocks side to side, that's probably top or bottom support pins.

I think it's a RC1 cap. The system is non-pressurised. Perhaps it's showing low because there was air in the system that has now worked itself out?

Gerry at Packards Southwest may have them as well.

And brake pliers work a jiffy for putting the bigger springs on. You can buy this entire set for $15 at Harbor Freight.

In both cars there have been many times I've pretty much had my legs over the back of the seat while being head down under the dash.

It's a yoga pose I call "upward facing idiot".

On one of the spare Jag cylinder heads I had, there was corrosion like that. It was aluminum. The head was stored in a basement on concrete that regularly got wet.

I'm sure the fact your car has a roll up divider window behind the seat complicates everything.

There maybe little tabs or loops coming out of the bottom back of the cushion where the screws are driven through and into the wood frame.

If you can't see anything, try reaching under there and see if you can feel the fastener or tabs/loops. Usually there is on each side.

Most seat back cushions lift up and off the back frame. There usually a couple of screws underneath that retain it to prevent it from poping off on its own. That's how both my 37 rumble seat and 54 back seat cushions come out.

Since these cars predate a heater switch, it was a manual deal where you turn it off in the summer. Otherwise the hot coolant give short circuit through the heater and back to waterpump. In the summer this can cause overheating.