At the dawn of the era of AC distribution, the earliest distribution systems were 1000V or 2000V single-phase, and 133Hz was a common frequency. It seems the transformers used on these pioneering systems were significantly different from what is used today. When the 2400V, 3-phase, 60Hz system was introduced (and which are now steadily disappearing in favor of higher-voltage Y-connected systems), the older 1000V and 2000V systems quickly fell by the wayside, and were practically extinct by the end of the 1920s. It seems the changeover happened so early and so completely, these early transformers are sufficiently rare that some collectors have a standing bounty for any surviving examples.