It's similar to this one..Hard to lap? It's not a Dalmore. My two Dalmore blue slurry very easily, but really with 2/3 strokes with diamond plate. But also the slates generally are not too hard to lap... Maybe it is a Llynn Idwal?
Post in thread 'Mystery hones... Let's see what you have' Mystery hones... Let's see what you have - https://www.badgerandblade.com/forum/threads/mystery-hones-lets-see-what-you-have.567924/post-10220465