All I can say for sure at this point is that Magic Key uses the same method as PassMe (which is also what is used in SuperPass) - so these likely will not work with the newest DS's that dont work with the others.
The first bit of the serial appears to not be a good way to tell which ones work or not, as every region has its own numbering system. As far as I have been able to gather, the EURO ones are NEH12****, and the US ones are NU40** and NU41** (with the US ones the last number in the box seems to tell which revision it is... [8] is apparently the not working one) - the JPN serial seems to be starting with NY4*** so...