Stop Block, insert anywhere

This handy stop block now fits some of the most common rulers.  It will work on folding wooden rulers, machinist rulers, flexible rulers, and common elementary school rulers.  Plus, it doesn't have to be threaded onto the ruler; the stop block can just snap into place anywhere.  The stop block clamps the ruler from the side, so it doesn't interfere with the measurement and it doesn't leave ugly marks on your ruler.

Included models:

• Machinist ruler, 25.8mm/1-inch wide to fit 300mm/12-inch rules.
• Machinist ruler, 19mm/0.5-inch wide to fit 150m/6-inch rules.
• Flexible ruler, 12.8mm wide.
• Precision 150mm rule, 14.9mm wide.
• Wooden/composite folding rulers also use the 14.9mm wide stop block.  

All stop blocks have the maximum width engraved and the lever has about 2.5mm of travel on a medium-sized stop block.  For example, the stop block marked 19mm can handle a maximum width of 19mm and can work with rulers down to just under 17mm wide.

No fasteners are required; the part just snap together.  This will be a tight fit, so a pliers wrench or squeeze clamp can be helpful.  When done by hands alone, care needs to be taken to keep the pressure directly over the hole.  Any wiggling or off center pressure can snap the small catches the hold the lever in place.  

Depending on how it's assembled, the lever on the stop block will be either righty-tighty, like a screwdriver, or some may find that turning left to tighten is more intuitive.

All credit to TORX1 for creating and sharing this excellent “Gauge for Folding Rule Carpenters Rulers, Meterstab”.