A faint design visible within a sheet can preserve information about paper production. It is a different kind of evidence from an inscription written on the surface by a reader or owner.
The Getty’s watermark terminology record describes designs incorporated during papermaking and seen through transmitted light. For a manuscript reader, the distinction matters because the paper and the writing have related but separate histories.
Working from a library’s documented image, record the visible shape and any catalogue identification separately. “A partial circular form is visible” is your observation; “identified as a particular maker’s mark” needs the catalogue or specialist reference supporting it.
Do not turn an approximate match into an exact date for the text. A sheet could be written on after it was made, and comparison may require measurements or imaging unavailable in an ordinary scan.
If the available image does not reveal a watermark, record that limitation rather than concluding that the paper lacks one. Do not attempt special lighting or physical handling of collection material without staff guidance.
Your useful result may be a precise question for the library: whether watermark information is recorded for this leaf. That question is stronger than an attractive but unsupported identification.
Editorial illustration from this site’s image library; not documentary evidence of the example or object discussed.