Treffynnon and White house burial chambers are identical twins, different only in their distruction.
Leaving the small village of Treffynnon to the south east I parked at a t-junction with barely enough room for one, went through the gate up the field through another gate and the chamber is to the right/east . The capstone is a perfect giants pebble for skimming across giant ponds and is dislodged over the entrance to the chamber. The chamber is full of field clearence and the obligatory black slugs. Considering the reconstruction of other Welsh monuments Anglesey's Ty Newydd for instance, it would take virtually nowt to set it right and enhance the site no end. I'd even lend a free hand too.
Three orthostats c.1.0m high define a rectangular chamber 2.4m by 1.5m. The partly displaced capstone is 2.0m by 1.6m and 0.9m thick. The chamber has been filled with field clearance stones. There is no trace of a mound or cairn.
Sounds alot like the picture of Whitehouse burial chamber further west, with luck I shall take a look shortly