Part number 0840-618513 is an Aero Easy Cover Keystock. This specific piece of hardware measures 6mm x 2-1/4 inches and is made of zinc-coated steel. Keystock is a small, rectangular bar that fits into keyways on shafts and sprockets, preventing rotational movement between them. In Aero Industries Easy Cover tarp systems, it's used to securely fasten components like sprockets to motor shafts or roll bars.