If you want a mechanical lock this is one of the few options - DoubleStar Push Button Folding Stock Mechanism w/o Boss

Very few options out there for side folding mechanisms that have actual mechanical locks. Most are friction locks and many work quite well. Stormwerkz comes to mind. However, if you want an actual mechanical lock, the ACE Doublestar is one of the few options available currently. Older versions developed wobble over time. The older version can be identified by the button being located at the hinge itself, where the newer model sold here on Optics Planet is located in the center and can be tightened if slop develops by turning the hexagonal portion of the button stem accessed by opening the folding mechanism.