Wester Six frozen helical

I have a super nice Wester Six, which is similar to a Agfa Super Isolette. I cannot seem to get the focussing helical unstuck. I've used naptha but no luck. Is there something stronger than that?
I dont want to force it.. parts would be impossible to find.

on a side note, the little NKK shutter on it is awesome. It needed a bit of lube and an adjustment for the self timer. Now its sprung to life. And it appears very well built. Any info about the company?

Penetrating Oil, apply sparingly.

It's a very common problem. I doubt that penetrating oil would work, but you might give it a try. What you have is that green gunk lubricant that gets frozen on the threads. Normally you have to take the shutter off the camera and soak the whole shutter/lens assy in a cup filled w/ 91% alcohol for a day or two. In a few cases I had to soak it for 3 or 4 days. Then loosen the 3 tiny grub screws on the front bezel that has the distance markings on it, and make sure you carefully mark where it came off. Best to do this over a laid out towel on a table w/ good light because sometimes they fall out. At this point the lens should just screw out. Make sure to mark the point where the threads release! or you will be in for lots of fun later. Now clean every teeny tiny bit of that green stuff that is left w/ Q tips and alcohol or lighter fluid, put a touch of lube on the threads (I use a touch of white grease and you only need a little) and put it back together,

I just assume any Isolette or Speedex that I buy has frozen focus because usually they do. Then there's the bellows pin hole issues. Wonderful cameras w/ great lenses once you sort them out, and they have very sturdy lens supports.

A bit of heat perhaps?

Well, I never got one loose w/ heat, but it's worth a try. Try a blow dryer on it. It can't hurt. I suspect that even if it does sorta unfreeze it, as soon as it cools down it will be back to square one. The penetrating oil actually may be worth a try too, but I found the easiest thing was to just take that one retainer nut off on the back of the shutter and soak everything in a coffee cup of alcohol. If there is even one tiny smidgen of that green Agfa lube on the threads it causes problems, so a disassemble and re lube fixes it permanently.

In post #2 I was thinking corrosion for the cause.

Grease is oil in a soap base. As a mechanism is used the oil is kept mixed with the soap base. Once the oil drains out from inactivity the soap base dries to a hard consistency that sometimes has to be scraped off.

90% Isopropyl Alcohol in an ultrasonic cleaner gets hot after several 5 minute cycles in less than an hour and causes paint/coatings of vintage cameras to bubble up and flake off but it will dissolve all but the worst dried out grease. It does not damage coatings/paint when cold.