I believe this trial is indeed an NG 985243 map ref. G V Wilson in his book 'Memoirs of the Geological Survey Vol. XVII Lead,Zinc,Copper and Nickel ores of Scotland 1921. page 146
gives the name as Loch Duich Trials with an assay result as follows: iron 55%, Sulphur 40%, Copper 0.25%, Nickel 0.8%,
with Silver 11dwt(penny weight) per long ton and Gold at 3.5 dwt per lon ton. Two Welsh miners were employed to drive a short level. Hope this is of help. Just as an aside I lived in Kilmore the village near the map ref given and found no signs of any trial near by. JeremyL