Vintage UNKNOWN Sharpening Stone (See Microscope Pictures) & Description Below

Description

Condition is "Used". Shipped with USPS First Class.
I know nothing about Sharpening Stones. I inherited these from my father who has had them for 50-75 years.
Stone Size (Length x Width x Thickness) = 8" x 5/8" x Thicknesses of 1/2" (at the ends) x 5/8" High in the middle.
Stone if straight on the bottom but the top is a gradual arch.
Has letters on it that might be BY CARBORUNDUM but I can not see all the letters.
#552 - Sellers Tracking Number to Ensure You Get The Exact Item Pictured in this Listing