SHS AK-series Selector Plate
The selector plate is a gearbox component that transfers the firing mode selection from the lever/switch outside of the gun, to the trigger switch inside of the gearbox. On the AK-series guns this has quite a few components, but the selector plate itself is on the left side of the gearbox, and is a plastic plate that acts on the cut off lever and trigger switch.
If your selector plate is damaged or worn, you can have instances of semi-auto only fire, or even full-auto only fire. In countries such as Germany where you are limited to semi-auto only, it is normally the selector plate that is modified.
This part from SHS is constructed out of high quality plastic giving it more than enough strength to make it last. Please note that as with all aftermarket parts, there may be some modification required due to dimensional differences between brands & guns etc.