Mint Cachet's?

I have a few items like this in my collection. This is a mint example of the official FD Cachet envelope for the Weizmann issue (Scott #70-1). I also have the version with the stamps and FD cancels, but those are fairly common. I rarely see the blank cacheted envelopes come up for sale. I am also unsure how rare they really are. I don't see them offered as part of the new issue service either. Are these valuable? Probably not, but they are interesting. I will keep looking to see if I can find out more about these.