What is "x-patent"

The X-Patents are all the patents issued by the United States Patent and Trademark Office from July 1790 (when the first U.S. patent was issued), to July 1836. The actual number is unknown, but the best estimate is 9,957. The records were burned in a fire, in December 1836, while in temporary storage. No copies or rosters were maintained by the government at the time, leaving only the inventors’ copies to reconstruct the collection.