Given location is that of Cwm Leri No.3 Adit on Simon Hughes’ The Mines of Talybont list

There is nothing known about the history of this mine but it was probably worked in conjunction with Llewynadda or North Alltycrib at some time.

There are at least three levels ascending the hillside on the Cwm Leri lode. The highest is open and was entered some years ago and found to be in an actively collapsing state, more recent falls blocked it completely a short distance in. This is more an open gunnis going into the hillside than a level.
Middle Adit was collapsed and its cutting crossed by a banked up forest track.. However, recent Forestry operations caused it to open up. It was surveyed for NRW and found to be a blind heading of about 40 metres.
The deep adit, which is down by the river, has the remains of dressing floors immediately adjacent, however these are nothing at all to do with this mine but are associated with the unsuccessful Talybont Lead Mines 1910 operations. They were responsible for the driving of Pryse’s Tunnel and were working in Felix’s Shaft. (see Altycrib Mine)

There was a tramway connecting these with Pryce’s Tunnel although most of the material processed was from Felix’s Shaft, this being the shaft cut by Wilkinsons Level, ore being raised to that level by a small petrol engine, mountings are still to be seen.

The Cwm Leri deep adit is technically open, but I have it on good authority that the inner reaches are partially filled with fines which were pumped in from the adjacent dressing floors; this now forms a sort of quicksand. This makes it unsafe to enter.
An attempt to explore it was made years ago by the late Simon Hughes and friends but they had to give up.

Map refs
Top Level SN 6472 8918
Middle SN 6478 8913
Deep Adit SN 6480 8907

No official access, NRW controlled land