I think what you have there is a manicule. It was a little mark a reader would draw in a manuscript, or it would be inserted there for him/her by the author of the text, to highlight a certain section and make it easier to find later. Jefferson or the reader has drawn it to point out an important passage in the letter.

It was also sometimes called an index, ergo "index finger". They were used very early, and continued to be used into the 18th c. . And in print and in signage long after- the classic big sign saying " This Way Out" , with a hand pointing, is using the printed form of the manicule.

There's a guide and some examples here, and a brief history here.