I've heard that the old two-pin version of the BS-546 plug were sometimes used for DC.
You'd have one polarity at one part of town and if you moved somewhere else, or took .... a radio or record player to a friend's house....and they had a different polarity n their mains, you'd have to reverse the plug.
If you had a 3-pin plug, you'd have to undo the connections and reverse them.
Again, this is all just stories I've heard. Don't quote me on this.
