Spools of this nature with flat conductor grooves were made and sold by both OB and Victor They were generally intended for high voltage AC electric railway installations where additional wood strain insulators could be clamped to the spool . The metal clamp on this spool holds the end of a long broken rotted off wood strain insulator . this unit dates from a round 1908 and was used on a 6600 Volt AC overhead system. this spool dates to the end of Akron High Potential Porcelain Company production around the time OB gained full control of their output.