Aye, surface is very rough, you can see the milling marks on the block! In fact it looks like it's a piece from a surface roughness scratch guage
Brother is up town and is going to see if he can get some wet or dry paper, between 400-2000 grit according to lapping guides I've read, we'll see what abortion I can make of this, better put your tin hats on this could get messy!