They look pretty sturdy. It comes as a pair of two, including the buttons, and screws for it. You just have to take off your buttons, and put this on, without changing the screwholes. It can be used as a simple strap-button as well, without secury locks.
My Warwick bass was used, and I didn't receive any lock-tops, so this is why I had to order a new set - I think there should be an option to order only the detachable part of the lock.
I think it fits any kind of straps, but the sleeve is rather thick, so you may not be able to pull through a brand new strap at the first try.