There was a fire at the patent office in 1836 that destroyed a lot of patents and patent models. The only other copies were with the inventors. At the time, patents were not numbered as they are today. The patents known to have been issued, whether the surviving documentation was recovered or not, were assigned an X patent number. Only around 2,600 of the 10,00 X Patents have been recovered.
Astonishingly, from time to time an unrecovered X Patent turns up, even after 185 years! datamp.org has been involved with two such findings as information on X Patents can be found there. In both cases the great great grandson of the original patent holder found datamp while looking for information about the patent documents they had. They are 5,125X and 6,037X.
Even more astonishing, someone I know has been actively involved in the recovery of a X Patents and, get this, has documented her efforts! Be sure to check out the link below.
Hampton, Barbara J. (2021) “Stalking the Wild X Patent,” Journal of the Patent and Trademark Resource Center Association: Vol. 31 , Article 4. Available at: https://tigerprints.clemson.edu/jprca/vol31/iss1/4