820001BK - 4 pcs. of M4 BLACK anodized aluminium locknuts, commonly used as rim retainers on standard wheel axles (i.E. Tamiya). Their hex is the usual 5.5mm size, but the flange is only 8mm diameter, to allow use of MST or other make rims with wide design, which don't allow use of the common wide-flanged locknuts.