See: ftp.resource.org/courts.gov/c/F2 … 16-62.html
wherein U. S. Patent 2,589,509 was declared invalid, and the reasons provided. The text of the original patent does not identify the cartridge filler other than as being a “heat generating material” and that the heat produced is as a result of a chemical reaction (burning is a chemical reaction). Very little information about the cartridge itself is provided in this patent.
There is a good summary of what is and is not patentable, and it’s very interesting with regard to this device, especially the prior use of the same principle by another inventor.
Several prior art patents are cited, some of which may cover the cartridge or the heat generating material. I did not look those up. If anyone cares to do so, they are listed at the end of the information within the link provided.
I imagine these things are at present more rare than Walker Colts, but probably would not command anywhere close to the price. Most of them are likely in landfills. Something else to look for at estate sales and yard sales. I go to lots of those, but have never seen such a soldering iron yet, other than what I previously mentioned.
It would be easy to convert one of the later models into a clandestine gun. I wonder if that was ever done?