This is only the second gold mule that I’ve ever heard of. The other is a 1829/1823 G.B. 1/2 Sovereign proof gold mule.

This gold mule is from Ethiopia and is unlisted. Somehow the obverse die which struck (1931) Werks was muled to the reverse of KM-18. The dies were not aligned properly and the reverse die was also rotated. A spectacular and unique major mint error.