A lot of old canvases were reused like this. There's even a hidden treasure map under the Declaration of Independence that's only visible if you look at it from a special angle under a black light.
/s
This is a true story - when I took my son to the National Archives in 2020 right before the pandemic, the security guard sent my son on a treasure hunt in the rotunda holding all the founding documents. He found all but one of the treasures ;)