Well, from an interview with Roger Mayer
"Well, Jimi would buy half a dozen of these pedals, find one that sounded great, and then we’d mark it"
https://www.premierguitar.com/articl...yer_Talks_Fuzz
And
"The large variations in tone quality from unit to unit, radio pickup and high frequency oscillations, temperature causing tone quality to change all made life very difficult and it was not unusual to have to select from up to 20 units to find a really good one"
From
https://www.roger-mayer.co.uk/classic.htm