A more thorough explanation:

ex scriptis olim visis

Ex scriptis olim visis is a legal term that means "from writings formerly seen." It is a method of proving handwriting in court when a witness has seen other documents that are believed to be in the same handwriting as the document in question. The witness must have had further communication with the person who wrote the documents or have some other type of evidence that would lead to a reasonable presumption that the documents were written by the same person.

For example, if a witness has seen a letter that they believe was written by the defendant, they may be able to use ex scriptis olim visis to prove that the defendant also wrote a different document that is in question. The witness could testify that they have seen other letters from the defendant that have the same handwriting and discuss similar topics, which would lead to a reasonable presumption that the defendant wrote the document in question.