Logitech K400 Plus did not come with a USB dongle? Wait!! It's hidden in the box!! | THINKBOX
Logitech Unifying receiver - Wikipedia
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Logitech K400 Plus Touchpad Keyboard for TV connected PC
Wireless Usb Receiver Dongle 6 Channels Adapter For Logitech Unifying Keyboard K400/mouse Mx M905 M510 Repair Replacement - Instrument Parts & Accessories - AliExpress
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Short review: Logitech K400r vs Plexgear Mediatrack Ergo - Erik Moberg's personal homepage - photography, gadgets, DIY, and more
Genuine Logitech USB Dongle Extender for Unifying Receiver | eBay
Wireless Dongle Receiver Unifying USB Adapter for Logitech PC Mouse Keyboard~ | eBay
Refurbished: Logitech Recertified 920-003070 K400 Wireless Touch Keyboard with Built-In Touchpad for Internet Connected TV's - Newegg.com
Amazon.com: Logitech USB Unifying Receiver, 2.4 GHz Wireless Technology, USB Plug Compatible with all Logitech Unifying Devices like Wireless Mouse and Keyboard, PC / Mac / Laptop - Black : Electronics