Excellent way to start a letter. That will be going into my stock phrases for emails.
I wear out and abuse your patience;
Unfortunately boring answer: the person receiving this letter is a Baron (or whatever the correct equivalent of Freiherr is in other languages) and the person sending it is an untitled public intellectual.
Though there is also probably something to them also being from different cultural backgrounds on formality, and thus opting for the more formal address out of caution.
The sender is a native Spanish speaker. The recipient is a native German and Polish speaker. So French is working as a middle ground here.
Given the type of writing, I think using "vous"/"usted"/whatever German equivalent, was the standard in any "we are not very close friends" relationship. And on top of that nobility, but like⌠children would also use "vous"/"usted" with their parents up until the end of the 19th Century/early 20th Century. I have seen it also in documents between friends or acquaintances of the same social class.
It's not the case in Spain anymore, we barely use "usted" (with old people and with important people, like the prime minister), but in France children still use "vous" with adults, meaning that if you grow up around certain adults, you will be an adult using "vous" with other adults in contexts where most adults just use "tu" (source: being an intrigued Spanish with a bunch of French people of ages 20 to 50 where some used "vous" and some didn't and the answer lied in "we were friends in school" "we are university friends"). "Vous" in French is used even in contexts where it doesn't make sense to be used in Spanish, like titles in fashion magazines or marketing ("Profitez de l'ĂŠtĂŠ avec X"). But that was also the case in Spanish a hundred years ago, so I'm guessing what's happening with that letter is just average courtesy.
It didn't shock me at all, that's just how you would adress someone you don't know or you're not friends with in many social contexts in French *right now*, and being a historical document, even more so.
























