In “Prehistoric Preseli – a field guide” (2001 Atelier Productions) NP Figgis mentions the missing capstone from the eastern chamber is “known to have been used for a bridge”.
At the western end of the Cerrig y Gof field is a stream, and the road crosses it over a small bridge with an interesting name: Pont Heb Wybod (“bridge without knowledge”). Dyfed HER pages mention that it was recorded earlier as Pont y Wibod (“bridge of knowledge”)!
Anyway, there are lots of stones next to the bank of the stream, including one large slab. Potentially of more interest though is a further slab built into the embankment next to the bridge structure. Could this have been the missing capstone?