Congratulations! As there's nothing behind the mounting threads to grab onto on these, you need a flange or an iris mount. You could try making a tight-tolerance lensboard hole that you can thread it into, but I'd also tie a good cord around it and secure that to the lensboard, just-in-case.

The Cooke mounting rings were a standard (for them) 13 tpi, were usually engraved TTH, and came in quarter-inch-diameter increments, so you might look around a little before having one made...