USB HID Adapter Kit – similar to bluetooth keyboard adaptor from HandHeld Scientific, but can be bought as a DIY kit. Made and sold only for Japanese market so far : r/MechanicalKeyboards
Amazon.com: USB to Bluetooth Convert Adapter USB2BT Assembly Kit Japan Import : Electronics
USB2BT PLUS [ADU2B02P] [assembled] from Bit Trade One on Tindie
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HHKB Pro2 Type-S & SlimBlade with USB2BT - YouTube
贅沢 BIT TRADE ONE USB HID Bluetooth変換アダプタキット USB2BT 組立済み ADU2B02P trumbullcampbell.com
Updated] Wireless GH60
USB2BT PLUS [ADU2B02P] [assembled] from Bit Trade One on Tindie
Convert a USB keyboard to work over BLUETOOTH! | Usb, Keyboard, Bluetooth
Amazon.com: USB to Bluetooth Convert Adapter USB2BT Assembly Kit Japan Import : Electronics