Naniwa 220 grit lapping stone.

[Bayamontate]

For a while there I thought this thing was useless but recently I started using it paired with 200 and 400 grit SIC powder and it actually works really well.

 

[Oscroft]

For a while there I thought this thing was useless but recently I started using it paired with 200 and 400 grit SIC powder and it actually works really well.

Interesting - I've had one for some time and have also considered it useless.