Collection of sharpening stones (Park-Monroe Neighborhood)

My husband seems to have been Very into knife sharpening back in the day, and collected all kinds of specialized sharpening tools. These include four sharpening stones, three coarse/fine, plus one brace to hold knife at proper angle. To be honest, neither one of us is quite sure what the small (obviously well used) white lump in the little plain white box is, but it must be some kind of sharpening stone.