Anita will most likely be able to give you a more definitive answer, but I've understood that the label was in use from the early 50's to the late 60's, when it was replaced by a clear plastic label. According to Pina, this blue export label seems to have been used at the same time as the silver foil label with the stylized "ft" on it. Why one label was used on some pieces and the second label used on other pieces has never been clear to me. There's also the castle label that was used for imports to the US.
I hope this is a start.