The sheet you’re looking at includes a Fool’s Cap watermark: we can see the head of a fool wearing a bell-adorned hat and a seven-pointed collar with bells, and three hanging circles below.
Watermarks help to date a sheet of paper, and thereby give an indication when a specific impression was printed. Thanks to the systematic research of Eric Hinterding and other scholars, it is now extremely well documented which types of papers Rembrandt used to print his plates and which watermarks occur at any point in time and in which state. Once a watermark has been found and identified, it is possible to look up, with a high degree of accuracy, when an impression was printed. This information can be found in Hinterding’s book, Rembrandt as an Etcher — Vol II & III: Catalogue of Watermarks, published in 2006.
The Fool’s Cap watermark that can be seen here is exactly the one to be expected on very early — and hence particularly fine - impressions of Jan Lutma, Goldsmith.
Watermarks are most easily seen by looking at the back of the sheet with a strong light shining through the paper or onto the paper from a sharp angle.
Not every print has a watermark. Full sheets of paper, as they were manufactured by the paper mill, usually had a watermark on one half and a smaller one, called a ‘countermark’, on the other. Especially smaller prints, which would have been printed onto only a section of a sheet, are therefore unlikely to include a watermark. When there’s no watermark, it takes some experience to estimate the print’s age based on the structure of the paper. If the wire lines and grid structure are fine and tight, it is likely to be from the 17th century.
Oriental papers or ‘oatmeal’ paper do not include watermarks, but their use by Rembrandt is also well documented.