Mark M was used by Japanese Mint during 1930s and early 1940s.
M for Mint.
The earliest currently known time coordinate of mark M - March 1930.
The latest - April 6, 1943.
Showa Enthronement Commemorative Medal.
Luckily for us, dated packages can be found in the boxes of these medals...
japanese markm
japanese medal markm
japanese order marked mmarkm on japanese orders
marks of japanese manufacturers of badges
marks of japanese orders
osakamintosakamintmarkm
japanese awards and decorations
japanese order marked m
order of the chrysanthemum
order of the chrysanthemum marked mosakamintmarkm
supreme order of the chrysanthemum
supreme order of the chrysanthemum marked m
大勲位菊花章
大勲位菊花章頸飾