[bombcar]

It doesn’t prevent observation - it provides evidence that it was observed. Likely with a ton of effort and careful application of steam, etc you could defeat it - but it’s more designed to prevent casual snooping.

So don’t think PGP encrypted but think a URL that records who views it.

[BrandoElFollito]

Why is it difficult to undo, then redo? Obviously one need to be careful (twice) but that's all

[bombcar]

The key is the adhesive which means you have to tear or otherwise undo it before you can unfold it.

I suspect that much of it was simply “how can we fold the letter” taken to extremes as a form of individual expression.