Dunlop Straplok Black Dual Design

I've been using these on all my guitars and basses for years now and I couldn't be happier. I actually started out using the "Flush Mount" version on 1 guitar and 1 bass when I first discovered the system, but went on to use the "Dual Design" on all successional instruments, because they make it possible to still use "regular" straps and of course because they're much easier to install and the mod is completely reversible.

What I like most about these locks is that there's no screwing around (pun intended) - apart from the screw that holds the pin of course. The counterpart that you attach to the strap is locked in position by a spring-lock washer that won't come off unless you use tools (like a small screwdriver) and a lot of force.
And should it not hold sufficiently anymore, because you have taken it out and put in in again (to adjust a "traditional" leather strap, to put them on a different strap, ...) too often, you can get replacement washers for a few cents at pretty much every hardware store. I've never had to replace one yet, though, and I've changed strap lengths etc. quite a lot over the past few years.

Also, when connected to the pins, the locks can freely rotate 360° so you're absolutely free to move. And you have to push the button (protected against accidental pushing by the collar) and simultaneously pull the lock out of the pin to separate the strap from the instrument, which makes it pretty much impossible to open these by accident.
I haven't had a single incident in about 6 years (using them on 11 instruments).

I used to use schaller strap locks on all my guitars but I bought a bass that had these on and I loved them so I bought them for all of my instruments. It is great that you can use a regular strap if you want to. The only drawback is that the part that goes on your strap is a bit difficult to fit, though I eventually got used to it after doing a couple of straps. Once they are on it looks like it would be a pretty tough job to get them off again! The only thing I don't like is that if you are using this and you do the usual trick of passing the cable through your strap (to protect from standing on your cable and pulling it out of the guitar) then because of this design, the strap does not tightly hold your cable next to the guitar. The strap attaches to the end of the strap button so that leaves a gap. It's not really a big deal but is something to be aware of. I have not had any problems because of this.