Curious this - at Oakeley, and it's precedents, the open workings were not referred to as twll, as far as I am aware, they were always sinc. There was Sinc Fawr - the lower and middle quarry open working, and there was Sinc Fach, the old upper quarry open working. Twll was a hole, as in a drill hole, or referred to the underground. e.g.
baril twll - the socket of a drill hole.
ceg twll - the mouth of a drill hole
dyn twll - an underground man
twll on its own is described as a mine, or a shot hole.
This info from Termau'r chwarel, the list of terms published in Caban
The map is the territory - especially in chain scale.