Do you know what this is?
No- it's blotter paper!
Back in the days before ball point and fountain pens, when you sat down to write a letter you would insert your dip pen (quill or metal-tipped pen), into a pot of ink, and before writing could commence you would need to blot the tip of the pen to avoid drips and splotches from marring your work.
Dip pens are still used today by calligraphers and the tips are called "nibs".
Example of a dip pen:
Close-up of a man writing with a quill pen. Image: Thomas Smillie, 1890, courtesy of the Smithsonian Institution.
Example of writing with a poorly blotted pen:
Sample letter from Perkins' correspondence collection, 1857, showing a preponderance of ink blotches.