The plan and section in Alan Holmes, 1999, Slates from Abergynolwyn p. 57 appears to show an incline down to floor 75B. I had thought this incline was underground and was the only access to 75B. (Holmes states that the eastern and western parts of floor 75 were never connected.)
According to Holmes' Introduction to the book (p. 11), the plan and section are based on his own c. 1976 underground surveys. Other than than the approximate line of the incline down to 75B shown on the plan and section, I've not found any mention of that incline in the book, although he does say (p. 72) that floor 75 is "completely inaccessible and is filled with water and rubble."
It's not obvious where Holmes found his information about the incline. However, I note there is an item (Z/DDJ/46/1077) dated 1963 in Gwynedd Archives described as "Letters John Snell, 60 Southwood Lane, Highgate, London to James [?Boyd] re Bryneglwys quarry and McConnel family. Enclosed Rough plan and cross-section of Bryneglwys quarry." I wonder if that was the source of Holme's floor 75 information.
Dave