“There’s a fake one that was made for one of the Plantagenet kings, Edward III I think, when he created the Knights of the Garter.”
I am thinking that the ones who theortically found it would have the awareness to take that into consideration.
Catholic priests also would mention Arthur.
Catholic priests also would mention Arthur.
~~~
I think that’s your best chance for historical leads, rather than relying on sources that have some composite characters both historic and fictional.
During the dark ages almost everyone was illiterate except for the upper classes and the religious class and monks did a lot of work recording events and genealogy. What survives is probably buried in the Vatican archives.
As I said, repurposed for different agendas at different times.
https://www.heroofcamelot.com/history/gildas-and-bede