The reader replied:
Thanks. I have been researching this. What started it is some sources that say the two blade plug we know today was invented as a way to join Christmas tree light strings together.
[What follows is a quote from another site:]
In fact, the bladed wall plug that we are familiar with today was actually a development of a device that was originally used to facilitate the interconnection of stings or festoons of Christmas lights. Some prototypes of this device were in use as early as 1917, and it was patented as the “Tachon” connector in 1924. The 1924 Tachon started out as a screw-in type of connector with a safety cover but soon evolved into the two parallel blade type.https://www.necanet.org/about-us/news/n ... mas-lights
This statement is copied other places on the web. It is not true.
The plug we know today NEMA 1-15 was invented in 1912 That is about 14 years before it was used in Christmas lights. It is quite possible that it was made more popular by being seen in Christmas lights.https://www.plugsocketmuseum.nl/NorthAm3.htmlhttps://www.oldchristmastreelights.com/ ... page_2.htm
Now I need to look for a Tachon connector with cap for my collection.