The National Gallery of Wales labeled this image as an engine shed but it appears to be a mill used to produce slate roofing shingles at the Braichgoch Quarry. Note the stack of shingles in the foreground. There is an overhead pulley and belt system in the rafters which would be used to drive slate finishing machines. There are several Greaves slate machines located on the right side of the photograph where the men are standing - 1885 Photographer? - National Library of Wales.