FreeBT is an open source Windows Bluetooth stack implemented by Antony C. Roberts http://www.freebt.net.
This is original FreeBT implementation with a number of my changes in user space code. My changes were initially released under GNU GPL v3 license for the Bluetooth for Microsoft Device Emulator project https://github.com/ten0s/bthmsdevemul | https://code.google.com/archive/p/bthmsdevemul/ and re-licensed here under the FreeBT license.
You can still download compiled FreeBT USB driver from https://code.google.com/archive/p/bthmsdevemul/downloads.