It's hard to really tell from digging around the US patent office, but I see a 2 prong NEMA 1-15 plug mentioned a year earlier than the crowfoot 3 prong Aussie style plug. [Linked Image]
Quote
Next question is, was the NEMA 1-15 invented before or after this? If before, I can see why the 3 flat pin plug did not become the U.S standard (much annoyance in not being able to plug in existing appliances with 2 pin plugs).

Looks like this is the case, that crowfoot outlets post dated the 1-15 and the crowfoot didn't allow the use of 1-15 plugs. I did see later on a 1-15 outlet with a 3rd flat pin, looking like |_| but as we know, it didn't get accepted.