Hi,
there is not much
documentation available about Baigysh opticals. All I can tell you is that your 6U works from 6V to 9V. Therefore a standard 9V battery connected in the proper way will do. However this unit uses a 6V battery, that is the reason why you have two slots.
The 6v coin battery is a russian Krona battery, and can be purchased at a camera shop or online, from manufacturer.
See:
Baigish 6U, order battery writing to
[email protected]. They can also give you the full manual.
Regarding the green light, either you connected the battery in the wrong way, or the monocular is defective.