This is an experiment... I have been carrying around a box of Mint Entire envelopes from the old family home, and they really should find proper homes. This is a batch of 21 wrapped in a strip of notebook paper, with notation on the back of the last one:
U538c Du4
They have taken a non-flat set from storage, but adhesive is not stuck. Smell is light mustiness with ancient whiff of Kentucky cigarettes <sigh>. I see these go for around $1.75 in singles, so priced accordingly (with free shipping)