After several accidents with suddenly disappearing retainer nuts of strap 'lock' systems, resulting in either sudden drops of bass causing dents or a twisted wrist/elbow, I decided to get rid of them and return the original knobs.
These 'blocks' are a far more secure system, no more rattle of metal parts and the blocks sit more tight. Looking better too. Of course YMMV.
For what they are, they are rather expensive but better than beer bottle washers and cheaper than strap locks. Now I always carry a spare pair.
Main drawback is flexibility: either leave the strap on or be prepared to fumble. And it sais FENDER all over, you must be able to handle that ;)