I've just checked a couple of my chucks, and yes it is 3/8" x 24 and 1/2" x 20. I've made enough adaptors for tehse things, I should have spotted it.
A very ingenious use of the nibbler, having cut a few inches off the end of a stainless steel sink with a hand cutter (the type that make s a coil about 1/8" wide) so it would fit in a camper, I can see many benefits.
Neil