That's what I wanted to know. I was hoping I could retrofit a couple of my existing cords before the Christmas lights go up next year.The piggyback style plug and the receptacle you posted are molded during the cable manufaturing process.
You can buy cables with that style plug on them at many places.
I don't recall ever seeing the plugs that you could install yourself.
I probably have a well labelled box of them in the garage or attic.not likely to find retrofit end IMO, but they are available as a molded pc for sure. you could jury-rig something ...
does anyone remember back to the 486 computers that came with power cords like that?