The platform/terrace knob was common before the Qin Dynasty (221-206 BCE). To increase literacy the mainland Chinese have, since the 1950s, used “simplified” characters. As a result one simplified character may be used for more than one traditional Chinese character, leading to confusion. One modern mainland Chinese author translates this knob shape as a “table knob” because the traditional character 檯, pronounced “tai,” means table or desk. The traditional character 臺, also pronounced “tai”—and the one that should be used for this knob (as can be seen by the shape), means a platform or terrace. Both use the the simplified character 台 pronounced “tai.” Since the two words are pronounced the same and differ only in the addition of the “wood” radical for the word “table” I suppose it is easy to see why they chose the same simplified character for both. In spoken context they would probably not be confused.
- Pinyin: tái niŭ