The only thing I'd add to this discussion is that all of Hubbell's plug designs were originally rated for up to 250v (6A). The parallel configuration was officially given the rating '125v' in the 30s, and the tandem plug was unofficially left implied for 250v until the 50s, when it was finally given that rating officially, a NEMA rating it holds to this day, even though connection with it on 240v has been "banned" since the 60s. Because almost all general service electrical connections in the U.S. were 120v, almost all tandem plugs ended up being connected to 120v. But the tandem plug was never officially made FOR 120v.
Most general purpose, light duty electrical connections in the U.S. have always been 110-125v, but in the late 19th century into the early 20s, there was in fact some 220v, too. No standards on plug types in those days, so the tandem and parallel blade plugs were used interchangeably as available. As well as a couple other Hubbell designs which were unpopular and soon dropped. Even though the plugs weren't assigned official voltages at this time, I do believe Hubbell's intent in creating multiple plug configurations was ultimately to account for the differing voltages in use at the time
