HP 41 archive web site.

[Thomas Okken]

Are you sure about that URL, https://hp41.deringenieur.net/ ?
There's nothing there, even if you use http instead of https, and according to the Internet Archive, there never has been anything there, either.

This one looks more likely: https://malte.deringenieur.net/

UPDATE: No, sorry, my mistake. There is something there, as I was able to verify by googling "hp41 deringenieur net". The first hit was http://hp41.deringenieur.net/file/HP-41CV_Repair_DF.pdf
I don't see how to browse that site, though, and apparently, neither do Google or the Internet Archive. But perhaps Malte can tell you; his contact information is at https://malte.deringenieur.net/uber/impressum/