Late 18th/Early 19th C. Sharpening Stone with Marvelously Carved, Monogrammed Cover

My favorite find of the week, in southern Maine--I am completely charmed by this late 18th/early 19th century American sharpening stone, singular in every way.  Making use of a very irregularly shaped piece of stone, which dictated the form of everything else, it feels much akin to early fragment mirrors. The stone was fitted into a quarter-sawn piece of wood (walnut?), with carved out recess to accommodate it, and then a cover was carved out with the same irregular form to fit perfectly over top. Then the monogram WM carved into the lid--as clearly this was a precious thing, not to be lost. Wonderfully specific and personal and early American.  4 1/8" x 1 5/8" x 1 1/2" t. Wonderful much handled antique condition, fantastic patina. One small split along one short age, pretty minor and not presenting issues. Lid fits very nicely and snugly over the bottom half.