Do you have any chance at getting in touch with a palaeographer who might be able to read it for you? If you know the date in which it was published, you have a good shot at getting some help from someone familiar with the era's secretarial scripts.

Does the archive have an accession number? I would consider using an identification number supplied by whatever archive you are looking at. Similar to the way cuneiform tablets are cited, by their number and collection. The main point of citing is to 1. Give proper credit to the originator of the idea, and 2. Ensure that other interested parties can find the same information that you did.

"Sine nomine" is for when the author is unknown. It's difficult to judge without knowing the original document or the subject of your work. Is the name simply unreadable or are you specifically having problems reading it? Outside of the bibliographical reference, unknown authors are often simply refered to as "unknown author" or "unreadable name".