The polarity is flipped from one side to the other, thus it likely won't be listed...

This is reminiscent of the double adaptors made in Australia until the 1950's that reversed the live and neutral pins on one outlet. But, at the time there was no official polarity given to the two angled pins. Convention was to always treat both pins as live (hence the double pole switches in portable appliances at the time), so it wasn't really that much of a safety issue.
This US example looked illogical until I read the patent; apparently the design is intended for DC use and saves having to rewire the plug the other way round if the appliance doesn't work on a certain polarity (eg. radio used on 120V DC mains).
It makes you wonder what the point of polarised sockets is in the first place if you can defeat it just by plugging into the adjacent outlet.

We had crowfoot outlets that look like they did this as well in the USA. But I used a continuity checker function on my DVM and saw that they crisscrossed the connections inside so both outlets would be correctly wired. I posted a picture of such a duplex outlet as a separate thread in this forum.

OK, had to register, I love that outlet! But it simply does not make sense that DC loads of the day would need polarity reversed, unless there is an item I can't think of at the moment. I collect antique radios, and radios that plugged into the mains outlets did not come on the scene till the late 20's, and AC/DC radios did not come around till the early 30's. Universal motors, lights, heaters, don't care about DC either. Were AC plugs polarized by 1922?

Robert