Basically, opening centuries-old letters that are sealed with this technique is usually a destructive process: you might end up rendering some portions of the letter unreadable.
As such, many of these letters have never been opened! They might contain interesting things, but we have no idea.
Some researchers figured out a way to "unfold" X-rays of these intricately-locked letters, to render the letter legible without having to actually open it! It's a pretty cool technique.
That articles leads me to one of my kinds of YouTube channels, and my favor type of little corner of the internet: A vast array of oddly specific and niche knowledge I had no idea I wanted
Some of the things we've done with old letters are fascinating.
I remember when they figured out how to use spectral analysis to 'see' the solvents that soak into paper from the ink, allowing them to read words that had flaked off due to the ink or the paper delaminating, especially at the edges of paper.
this is one of those technologies - drop a letter from centuries ago onto an x-ray and let it get displayed on a computer screen - that will really make future tech look like "magic"
It's pretty easy to non-destructively lift a wax seal off of paper, with a sharp, hot knife and a bit of practice. You can then read the letter and reseal it with a touch more hot wax or the back of a heated spoon (to melt the back of the original seal).
Not that I've ever done that, as a courier, in a live-roleplay game, ever. Repeatedly ;D
Depending on the security level of the letter, of course, a non-letterlocked letter might be pretty readable even if sealed. A simple letter where the seal authenticates the sender but doesn't protect the contents might simply be folded in three and sealed closed - you can bend and flex such a letter without breaking the seal to read most of it. A more important letter being _protected_ by a seal might be folded into an ersatz envelope and then sealed on the join ... but that's most of the way to a basic letterlock.
So, like all communications there's a tradeoff between complexity and security, and whether you're using the seal merely to authenticate the sender (which was pretty common) or also to protect the contents.
To answer your question, the lifting technique I refer to works well on flat paper and variably well on non-flat paper. Something as delicate as a letterlock, I'd be seriously concerned about my hot knife nicking or tearing a part of the lock that I can't see under the seal ... and /that/ would be obvious.
So it's not so much that the seal holds the lock closed, as that the seal obscures the lock to the point that opening non-destructively is much harder.
It was a ~1000-person national level game in a fantasyish setting, and one of the game elements was that you were explorers and colonists in a strange new land, and you could send messages to your supporting factions "back home" to request support or supplies or whatever. Those messages had to be carried by a ship belonging to a trade house, and any supplies that your supporters sent you had to come back by the same method, and all the trade houses were represented by player groups on the field...
So yeah, a group of players would write to their "sponsors", and my group (as one of the trade houses) would open their letter, see what they were requesting and then forward the letter to the game organisers. The organisers would play the part of the remote sponsor, and send a letter back, which we'd open and read to see what orders the players were being given and what support they could expect (then we'd reseal the letter and deliver it). This positioned us super well to sell them exactly what they needed to achieve those goals, at an only slightly elevated "rush" price.
We didn't open every letter, and we didn't scam everyone - we made most of our money legitimately. But it was a fun side-game.
Rumor has it that Cardinal Richelieu's men had ways to read wax sealed documents.
How many of these documents have multiple layers of meaning embedded in them? Steganography, euphemisms, inside jokes, shibboleths, what have you. I wonder how many things these letters say that we simply cannot read.
Looking at the links in the Wikipedia article I discovered this wonderfully “old web”-style page with various envelope folding guides[1]. I found it so endearing that I sat down and tried a few of them out, which was great fun.
I don't know much about the device you describe, but fastening papers with something like this is probably way better than staples if the goal is long-term storage. Staples and other metal fasteners are pretty destructive. The National Archives offers guidance for fastened document preservation, for example, here: https://www.archives.gov/preservation/holdings-maintenance/f.... Staples can add a lot of unneeded bulk to records, too.
There still is. We have a couple of these in my house. It's not as effective as a metal stapler, but it works well enough for things that we don't need permanently connected (plus it's much safer for my 6-year-old kids to use).
There are crimp-type devices, and there are ones that cut a slot and tab. The slot and tab devices were patented in 1910. The main one was the "Bump Paper Fastener", which shows up on eBay now and then. So that's automatic letter-locking.
Basically, opening centuries-old letters that are sealed with this technique is usually a destructive process: you might end up rendering some portions of the letter unreadable.
As such, many of these letters have never been opened! They might contain interesting things, but we have no idea.
Some researchers figured out a way to "unfold" X-rays of these intricately-locked letters, to render the letter legible without having to actually open it! It's a pretty cool technique.
The underlying paper is here: https://www.nature.com/articles/s41467-021-21326-w