This group of fence insulators were all made by Porcelain Products Inc. I think it's neat that P.P. used a Lt. blue skyglaze on their later made spools and nail knobs. I guess to bring attention to them or mabey was just an optional color. The brown nail knob is unmarked but as you can see the same as their 5 1/2 knob. They are marked: P.P.Inc./5 1/2/U.S.A./62,Lt. Blue, NN Brown, P.P.Inc. cobalt, P.P.Inc./5/U.S.A.,Lt.Blue, P.P. Inc. cobalt, P.P. Inc. U.S.A. No. 20, white.