2-Prong Plug in the US & Mfg. Date

Hi everybody,

I think I should be able to better estimate the era of electrical items intended for the US market by looking at its two-prong plug.

So here are a few questions about 2-prong plug in the US:

1. Around what year did two-prong outlets with same-sized prongs cease to be made? (i.e., if the prongs on a two-prong plug are the same size, it was probably made before ___.)
2. Besides identically-sized prongs, what other clues are good for collectors/dealers to know?
3. Are there any good websites that expand on this?

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Update:
The first question is my most pressing issue. I picked up a lamp that I think might be a very good example of Mid-Century Modern. I would like to make a better estimate of its date based on the fact that the plug has identically-sized prongs. Based on things I am reading, I think it might be sometime in the 70s for question #1, but I'm not sure.

 

only items where the plug has not been swapped out !!!

toasters plug likely stays original. where as a lamp plug..... u can get those anywhere...for any time..

 

A lamp has never needed different size prongs as it has no polarity

The bulb socket may be more help in dating

 

Polarized outlets came into usage around 1962. Devices took longer to adapt, so if it has an original polarized plug is at least 1962. Unpolarized were still used for quite a while after that. The radios/TV's etc with a "hot chassis" started to use them first if I recall correctly.

 

Would anyone venture an educated guess to narrow down "quite a while". Say just as a random example, how commonly were products being manufactured for the US market in 1978 that had unpolarized two-prong outlets?

 

can we see the lamp ?

 

https://www.ebay.com/itm/155753310789

this guy sez 1950's , made in Canada

 

https://www.ebay.com/itm/133876109343

this one sez 50's..60's.....

 

maybe an electrical vintage parts site ....could be helpful..