I recently found this Japanese 1 sen and trust me, it's quite difficult to obtain in a decent condition:

It's a Japanese 1 sen dated Showa 14 (1939)
A normal coin of this year would be something like this:

The difference is with the character four. The left stroke is slanting towards whereas a regular coin of this year has a "box" on the left side of the character "four". Do try and keep a look out for it. Maybe someone here has such examples? :)
Japanese coins from the Second World War era are pretty interesting as there was many experiments released to the public, such as changing the planchet and trying out porcelain coins (at the near end of war) and reducing the weight of some. I don't have a full set of them which may be of some challenge. :)