OmicronLab (Official Developer) Direct Download: https://www.omicronlab.com/avro-keyboard-download.html (Scroll down to "Previous Versions" → Select "Avro Keyboard 45.1")
Searching for "Avro Keyboard 4.5.1" can be tricky because while version 4.5.1 was a significant milestone in its history, it is now an outdated version. The current stable version for Windows is avro keyboard 45 1 version download verified