In reply to JimR:
I've got 2 sliding nuts:
- A Faces one, which has a micro cam-style slider in a captive groove on the main nut.
- An HB brass one, which is just a wedge sliding in a groove on the main brass body.
One of them is called a Slug, but can't remember which one!
I've definitely carried them, pretty sure I've placed them at least once, but never fallen on either.