No, that's not true, there are workings above 2 accessible but not a huge amount.
The North Vein is really a separate little mine cut by the Oakeley fellas in the 1920's.
The North Vein was quite productive on the Oakeley side so the decided to drive into it from the Cwmorthin side, as it hadn't been tapped yet on that side of the valley. It was put it on the same height as Floor 5(?) Oakeley with the possible aim of driving all the way through into Oakleleys existing Floor 5.
I guess the slate wasn't as good on the Cwm side because although a number of chambers were cut (7?) none of them got very big and they seemed to give up on it before too long. I suspect by the time the water dropped in the mine proper to open up the lower Cwmorthin floors, that looked a better proposition and the men worked there instead.
I'm sure GI will correct me if any of the above is wrong.
So the North Vein workings on the Cwmorthin side are a totally separate but small series of tunnels and chambers. There is one adit, but a roofing shaft comes down from the hillside (rope length: 40m).
The mine is on the inspection circuit, we include it because it's useful for things like this sometimes. Just somewhere small, safe, easy to get to, no drops, hard to get lost in etc. Good for filming where you just need a few tunnels etc.
Beneath my steely exterior beats the heart of a dashing hero