Spot the Kellow indeed!
My understanding is that William Kellow (Moses' father) was agent at Park Quarry (not Parc Mine) 1874–1892.
Joseph Kellow (Moses' uncle) was agent and quarry engineer at Conglog Quarry about 1870-72. (Conglog Slate Quarry, Celia Hancock & M.J.T. Lewis, 2006).
With regard to the "machinery ... of his own invention" statement in the original advertisement, I've not come across any patent applications in the name of Kellow other than those of Moses himself.
Dave