Stahlin REHPSERIES4. Robroy Hole Plug, Series: HP Series, 1-1/2 to 2 in Conduit, For Use With: Electrical and Instrument Enclosure, Fiberglass Reinforced Polyester, Smooth, Glacier Gray. Additional Description: STHL-HPSERIES4 HOLE PLUG SERIES 4. Catalog Description: Stahlin's Fiberglass Hole Plug provides an ideal combination of high quality protection in corrosive and outdoor environments and proven resistance to UV degradation..