@ used to signify French "à" ("at") from a 1674 protocol from a Swedish court (Arboga rådhusrätt och magistrat)
This makes sense considering that in french "a" and "à" are two common and very different words and "@" might be faster to write in cursive. In the doc it looks like the writer made the "𝓪" ending at the bottom right and, without lifting his pen, added the diacritic.