This particular solo was created before they made Fools for the 1971 album Fireball - it can, for instance, be heard on the recordings from Stockholm in November 1970 on Scandinavian Nights (aka Live and Rare), during Mandrake Root , and was later played during Space Truckin' (check out Made in Japan or the 1972 disc on Deep Purple in Concert, and the Copenhagen 1972 video.) It's also featured on the studio jamming called Guitar Job on Ritchie Blackmore: Rock Profile Vol. 1. This was recorded during rehearsals for Fireball.
Other examples of Ritchie using volume control feature the live endings to Child in Time , and the end of the Guitar Movement on Gemini Suite Live.