An old photo of Capel Garmon by Alvin Langdon Coburn. There’s a second and third photo of the tomb by Coburn on the same web site.
The date of these photos isn’t clear but, the tomb was excavated in 1927, so that gives us a clue (so long as it wasn’t restored before that). In the photos the site is neatly fenced off for public access, so it’s probably not been recently excavated. Coburn died in 1966, so the photos may well have been taken between 1930 and 1966.