M5GO BOTTOM is apparently the bottom part of M5GO which is a upgrade of basic bottoms, with a larger battery(500mAh),Analog Micphone, LED Bar , two more HY2.0 PORT(PORT B/PORT C)，and more. Its back is compatible with Lego holes, and the pogo pin can be used to connect to supported I2C devices (such as BALA) or magnetic charging bases.
Product arrived on time and in perfect order. Thank you very much.
The unit arrived and works. I'm a little upset that the original unit only lasted 3 months
Has to go with the M5GO big battery bottom. Very useful as you can keep the charging base permanently plugged in and stick (it has magnets) the unit on when it needs charging. There is a bright LED to indicate charging/finished.