We found this Sheaffer 444 in a second-hand goods shop. The barrel was stuck, but the finish was near mint. Not a scratch and stickered too (although the lettering had faded). The section had some scuffing from the inside of the cap but nothing bad.
After a long soak and some work with section pliers, I was able to finally unscrew the barrel. My suspicions were confirmed:
Somehow, the ink cartridge had leaked into the inside of the barrel, and the result was a rusty, gummy mess.
Anyway, managed to clean up the insides as best as I could and managed to sell the pen. A happy ending for all concerned 🙂