sharons25, many thanks for the updated images. You might delete the second image in the original post which shows the back of some envelope. Be aware that phones reuse image numbers so you might not get the intended image posted which I am guessing is what happened here.
Here is the major die type I that that person may have been talking about:
this is from an old Scott Specialized showing the die type that the cut square rarity could come from. However, the rarities are on the scarcer paper colors: blue, manila and amber manila which are pretty distinctive vs. the usual white or amber. These particular cut squares would only be from envelopes. If the color was not obvious from any scan provided, I would question the identification.