The famous phrase freeing the slaves in rebel territories is "then, thenceforward, and forever free."
So here we have the "forever" of the now-familiar "forever stamps" completing the idea "free forever."
Maybe you had to be there ... or have taken a lot of useless lit classes.
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Anyway, here is the last commemorative stamp:
Have 50 years really gone by since?
You could mail a first class letter with one of these.
Check out the price.