The Bendix blue band (Overdrive) 2-speed hubs always have a 20t sprocket tooth count. The Bendix yellow band (Underdrive) 2-speed hubs always have a 18t sprocket tooth count. The blue and the yellow hub shells are interchangeable, meaning you could use the Underdrive parts and the "Overdrive" brake arm in a blue band shell and pass it off as an Overdrive 2-speed... or simply repaint the bands blue on a worn yellow band hub. I've seen this done before by some unscrupulous eBay sellers. The tell-tale sign is that an Overdrive hub has a 20t sprocket and an Underdrive has a 18t sprocket.