Shop zinc plated carbon steel key stock, 3/8" x 3/8" x 1' from MDS of Michigan. Key stock is a general industry term for flat, rectangular, square, or otherwise-shaped bars of metal that are used in power transmission applications or to make other fasteners. Technically, keystock must be at least one foot in length and conforms to tight tolerances. This differentiates it from machine keys , which are usually products of keystock.