An interesting standing stone is to be seen on the southern slope of the Chevin above the town of Guiseley in the valley of the Aire (...). This stone is well-known to the small number of people who live near at hand. A similar stone is said to have stood at the head of Occupation Lane on the western end of the Chevin, and to have been broken up when the cottage was erected at that place. It is always called the “Bull Stone” and is said to be “lucky.”
Editorial Notes in the Yorkshire Archaeological Journal, volume 34 (1938).
Also, I read in ‘The English Dialect Dictionary’ (Joseph Wright, 1898) that a bullstone is a West Yorkshire word for a whetstone – which makes sense maybe as an explanation for (or even genuine use of) the grooves?