A more thorough explanation:

An X-patent is an early type of patent granted in the United States before the numbering system was established in the Patent Act of 1836. The name "X-patent" comes from the fact that an X was added to the numbers of existing patents to avoid duplicate numbers.

For example, if there was already a patent numbered 123, and another patent was granted with the same number, the second patent would be labeled X-123 to differentiate it from the first.

This system was used for a short time before the numbering system was established, and X-patents are now considered a historical artifact of the early days of the U.S. patent system.