You know what a workbench is like-things disappear then re-appear well this one is timely as regards this board.
It's a small-the coin is a quarter-unit picked up from Lee Valley Tools years ago maybe as many as 20 years.
Apparently for large scale wood saw blades they sold it as a knife sharpener-it does sharpen a knife but quickly removes an awful lot of material-when used correctly it leaves a very finely serrated edge quite unlike any other.
The proper technique is to apply it while using an anti clockwise motion from tip right to tang.
I think the steel itself has tiny circular striations (if that's the term)
around it they are difficult to see in the photo and little easier IRL.
Long discontinued it's just an oddity now really but OK to keep around for bench tools & low grade utility knives.