This stone is listed by the Gwynedd Archaeological Trust at the grid reference SH66601365, but they also list a second stone nearby at SH66591364 (notHafotty-Fach Stone II). I can see some scope for confusion identifying one from the other in the field as they're very close to each other. As I've failed to locate either, I can't really help clarify the situation. Strangely neither of these sites appear to be listed by the RCAHMW.
I've added the numeral to the name of this site. It's marked on the Ordnance Survey Landranger map.