I purchased these so as to be able to add a strap knob to my Thomann Guitarlele Standard. The strap knobs are well made and come well packed. The packaging is re-useable which is an added bonus since I only required one strap knob.
The Phillips head screws, that are provided with the strap knobs, are 3mm in diameter and rather short. For a ukulele style instrument or light acoustic guitar they will be fine. On a heavier instrument they would be insufficient.
To install the strap knob, drill a 2mm pilot hole to the appropriate depth. Make sure to drill straight! Lubricate the screw threads with some wax and then screw the strap button and leather washer to the instrument’s body. It is important to use the correct size Phillips screwdriver. Screw heads on these type of screws are notoriously soft and will easily strip if the wrong screwdriver is used.
Pros: Well made knobs, re-useable packaging
Cons: Short screws with Phillips heads. A hex head screw would be better.
In conclusion: An excellent value set of strap knobs for lighter instruments.