I found this coin in a junk box today. There were two other coins in there - a 1741 8 reales and an 1878 goloid dollar (I will not be posting pictures of them because they are magnetic Chinese fakes).
The 1889 Yen is not magnetic like the other two fakes. I do know that the Forty Thieves website has other dates of this type.
EDIT: I did a weight test on the coin. Genuine 1889 Japanese yen coins weigh about 27 grams. The coin in question weighs just 17g. Pretty damning evidence towards it being a fake. The other two counterfeits have similar weights. To make sure that the reading wasn't skewed, I weighed a real 1885
Morgan dollar which had a weight of about 27g. (The scale I use is an El Cheapo made in China scale)
Here are scans of the coin.

