The existence of a previously-unknown usurper in the Gallic Empire known as Domitianus II was first debated after the discovery of a coin in 1900. The coin was thought to be either a forgery or retooled, however. Two coins of the Emperor were found later that helped to confirm his existence. Here is a good article on Domitianus II and the numismatic evidence that proved his existence: https://antigonejournal.com/2022/10/domitian-ii-lost-roman-emperor/
We had a good thread here too about Domitian 2: https://www.cointalk.com/threads/15-years-ago-the-resurrection-of-an-emperor.332210/
Here is the remainder of the hoard on display in the British Museum when I went there a couple of years back. My photo of the Domitianus II was not successful.
