I don't think it was an engine shaft, I suspect it's much more likely to have been the hauling shaft for driving a sough along the sole of Orchard vein up towards Oxclose and so what you're seeing is a large hillock of driving rather than sinking waste. There are some other veins marked as being parallel on the barmaster's map, and interestingly, there's a "sough on vein" apparently feeding into the brook to the north-west.
More of the Op Mole material should become available as the Nash collection gets sorted out, Davetidza will be able to say more about that.
Follow the horses, Johnny my laddie, follow the horses canny lad-oh!