On the misprint...honestly we moan about G4 quality control but G3 had just as many issues. Missing symbols, part missing symbols, colour missed from symbols, misprinted symbols all turn up with reasonable regularity. However, she will be a cute addition to a collection as no doubt the misprint makes her a little more unique than all the other Sunshine's.
As for the serial number - I believe the 5 digit code works the same as with the G4 which is a factory/line id number [9], then the production year [3] and finally the day of the year the pony was produced [341th].