Looks like these folks rebuild them:

https://www.my-classic-car-trader.com/product-page/distributor-vacuum-advance-rebuilding

Buchannan Auto Electric in Oakland (510) 452-4025 was able to find one for me, don't know how or where.

I had never had any trouble with my trans cooler, but as part of rebuilding my engine I took it off and was cleaning it and preparing to paint it when I noticed that a large section of the outside had become paper thin from rust, and in fact broke through when I was cleaning it. I got one in much better condition from Max Merritt IIRC, and sent the core to them, perhaps they would be able to repair it for someone in the future. Ross IIRC recommends going to a modern (air) cooler on the radiator, because if the internal element (the part where the oil circulates) in the stock unit fails you might never know.

I am not sure why, but the website keeps hanging up whenever I try to post.

There is a directory of the major suppliers including those mentioned in the Main Menu on this website, to the left of your screen normally.

The Packard Club (www.packardclub.org) has a downloadable PDF vendor directory.

Many of the local Packard Clubs such as Northern California Packards (norcalpackards.org) also have their local list.

Looking at the service manual those are 10mm plugs.

So from this post:

https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?topic_id=24501&post_id=236196#forumpost236196

It says

AC MC-8
Autolite 3126
Champion UY-6

Maybe if those work for you, you could add a listing to the cross-reference on this site. My car has 14mm plugs so cannot directly check.

Kanter Auto Products sells this paint

www.kanterauto.com

As does Max Merritt

www.packardparts.com

As does Bill Hirsch

https://www.hirschauto.com/Paint/departments/1/?gclid=EAIaIQobChMIzL-tpZO--QIVuBPUAR11bgoeEAAYASAAEgJldvD_BwE

That goes to the vacuum reservoir tank inside the fender, which then goes to the power brakes. That way you still have a few vacuum-assisted applications of the brakes even if you lose engine power.

What did you paint the manifold with? Intriguing...

ClassicRadiator.com advertises in the Packard Club newsletter.

There is a pair of 1951-54 radiators for sale right now on ebay.

What was called Antique Auto Battery (closed in 2018, though someone is using their website address) used to produce this 6/12 battery which was two 6V batteries connected via that solenoid above, which activated when you hit the starter button, and only sent the 12V through the starter. It was designed to make up for cars with worn starters, etc. Not exactly the application you are thinking of, but something to be aware of. Each of the batteries was normally charged as 6V batteries in parallel, and ran the accessories and everything that way when the solenoid was unactivated.

When I got my car it had one of these, and it worked o.k., really made the starter rip (maybe bad for that!) but the solenoid would sometimes stick and I'd have to get out of the car and hit it with something.

Eventually, those units becoming very expensive, I purchased a regular 6V battery and just re-did all my connections, and the thing started fine just on 6 volts, and eventually I rebuilt my starter and now it is even better.

(I am specifically speaking about the window motors, why get NOS ones or adapt something else when you can get the old ones rebuilt).