These T-Slot nuts are featured with a spring-loaded ball, which prevents them sliding out of the profiles. You can both slide them in from the side, or click them in halfway trough. They have a raised middle part, to help having the thread always in the middle of the slot opening.
Of course they fit all Treq sim racing rigs and accessoiries, but they are also compatible with all other 40-series type profiles.
Available in M5, M6 and M8 thread size.