When was your previous date? How long do you think they used National boxes for?
I don't know how early but for some reason I wouldn't be surprised by 1934 for rainbos, even though I think that Picture Marbles on older style Peerless Patches were still being sold after that.
I'll have to dig a little to see why the year 1934 comes to mind.
Would be interesting to see a string of National boxes and try to get an idea of the changes in the boxes with time.
And of course would be nice for Mike to give us some years to anchor the discussion with, if possible.