I picked this up in a charity shop today for 20p
It's a Watchmakers Loupe magnifier. I can safely say I can see things on my edge I never saw before. I have got my knife to borderline surgical sharp.
I never thought about magnifying my edges on inspection, but it turns out to be a wise move. Great thing is this magnifier is tiny and very strong. Unlike a normal magnifier, this was designed to be used for close inspection use.