I attended (virtually) this year APS's summer seminar. One of the highlights for me was Ron Lesher's BOB program. In that program, he discussed embossed revenue stamps from the late 1700s, early 1800s. I was so intrigued by the history that I acquired a 1798 promissory note with a 30-cent Pennsylvania embossed revenue stamp from Richard Friedberg. It's RM179 in the Scott US Specialized Catalog. It's a simple example, but it packs in so much history. The borrower was the firm of Nottnagel, Montmillon & Co., a Philadelphia merchant, and the lender was Louis Clapier, a prominent Philadelphian who left a rich history for the city.
I would love to see other embossed revenue stamps!