Here are some of the Cook Pottery Co. split nail knobs I have showing the different two and single groove designs along with a unusual inner skirt design pattented by them US 919,386 and US 1,218,181. The black glaze one is debateable against Star Porcelain Co. as to who made the black glazed, grey porcelain nail knobs. As you can see I have an unglazed never used one with the grey porcelain body and a black glazed one. Star Porcelain Co. advertised a grey porcelain black glazed that they called Nu-Blac. I have a couple of Fleron Lightning Arresters that have the Nu-Blac and Star markings together, but I've only seen it on Fleron insulator products. Cook made insulators for Philco and other companies also just like Star Porcelain Co. Maybe they both had rights to use this formula. Possible? Dan Howards old magazine "Old Familiar Strains" (which is great reading by the way) gives more detail on Star and Cook in the December 1996 Vol. 3 No.6 issue on this subject. N.I.A. has 2016 PDF editions in their archives that you can download for free. It's a wealth of info on Radio Strains, Lightning Arresters and companies related. A great addition to anyones insulator reference library.