It appears that Miles Aircraft wanted to test their supersonic razor wings on a subsonic aircraft first. So they decided to use another Miles built aircraft. This was a regular general aviation aircraft called the Miles Falcon which had 130 hp engine and had its high lift wing which replaced by a wooden razor wing and a single piece horizontal stab.
An original Falcon is shown here:
Miles M.3A Falcon, G-AEEG (museumofberkshireaviation.co.uk)
The modified Falcon is shown here:
Miles Gillette Falcon L9705 (Print #9903761). Photographic Prints (prints-online.com)