These are one-touch connection connectors that allow you to connect cables simply by inserting the stripped cable while pressing the button. No soldering or tools are required, and they can be repeatedly attached and detached. They seem to be sold on Amazon and Mercari for around 600 yen each. I don't have a use for them, so I'm offering them at a super special price of 50 yen each. The terminal diameter is 2.1mm, standard DC jack specification (inner diameter 2.1mm, outer diameter 5.5mm). It's center-positive, with the red side being positive and the white side being negative. The compatible wire gauge is a maximum of AWG18 / 1.0mm / 0.75SQ. The cable fits perfectly when the insulation is stripped 8mm. The fourth image shows a combination with the DC male connector also listed. If you purchase the items also listed at the same time, I will discount the shipping difference, so please feel free to contact me.