Unique Two-finger Handle Mug made in JAPAN

I got this mug from my mother, and we both are interested to know more about it.

The greenish brown under glaze is matte while the black and white paint over it is raised and glossy. It is definitely hand painted.
The imprint on the bottom is very odd also. JAPAN is stamped by voided glaze; negative space. The hole in the letter P looks like a raised flower.

If anyone knows more about this piece and it's origins, I would love some feedback!