Norton Medium Crystolon Stone JM6 (61463685975). Official Norton IM313 Multi-Oilstone page lists this part as Medium Crystolon Stone JM6, 11-1/2" x 2-1/2" x 1/2". Crystolon is Norton's silicon carbide abrasive (store title spelling is Silicone). Official family copy: silicon carbide for fast sharpening; use with oil. Store URL still says replacement stone for the large multi-oilstone, which matches the IM313 spare. Specs from the nortonabrasives.com IM313 Multi-Oilstone Sharpening System page.

The gray silicon-carbide spare for the big three-stone box. Medium Crystolon, oil.