COLSON Casters have a zinc-plated finish with Zerk grease fittings. Slotted mounting hole centers. Swivel models have a double-ball hardened raceway.Wheel type codes are: FB—Performa rubber-polyolefin core with black flat tread wheelRG—Performa rubber-polyolefin core with gray round tread wheelPO—polyolefinPH—phenolicHT-PU—Hi-Tech polyolefin core with polyurethane treadSS—semi-steelPU—steel core with polyurethane treadMR—steel core with mold-on rubber tread