Before he fitted the three Burns Tri-Sonic pickups to his Red Special guitar which feature in all Queen recordings and live concerts, Brian made his own pickups with his father Harold’s guidance. He discussed them in an interview with Simon Bradley for the Red Special book in which they featured in Chapter 6: Revisiting The Past. You can read the transcript on the official Red Special website which is maintained by Simon:
I recently completed a project to make a replica set of these pickups following Brian’s original design sketch but winding them to try to achieve similar DC resistance (7.0-7.4 kOhm) and inductance (2.0-2.3 H) to a typical vintage Tri-Sonic pickup. I made a 17 minute video covering the project in which I discuss the pickups and wind one using my Stepcraft 2/840 CNC machine and small AC industrial milling spindle. The only additional piece of equipment I bought was a magnetic wire tensioner.
There is more information and some images in the article in my FAQ section here: