The refers specifically to a board design (R29) used in the MXQ Pro 4K line of Android TV Boxes, characterized by the LP3 (likely DDR3/LPDDR3 memory) and V2.3 revision markings on the motherboard.
Disclaimer: This information is for educational purposes. Always verify firmware compatibility before flashing.
Use a toothpick to press and hold the (usually hidden inside the AV port).