The circular locking knob is called a brake (or lock). Be careful you don't poke yourself in the eye getting a grip on these nasty things!

Whenever buying a cable release for the Pentax 67, take the camera along and fit the prospective release, as the shutter throw on the 67 is deep. I had a German-made release last November that bottomed out without firing the shutter. An humungous, clunky release from Hasselblad (curiously, Made in China...) did the trick, though I protested long and loud at the atrocious cost. Meanwhile, I've never lost the 60-T3 electronic release for my EOS1N, bought in 1994 and still firing.