From British Pedal Co:
The BPC MKII Tone Bender OC81D is a detailed replica of the 1966 PROFESSIONAL MK II Tone Bender pedals created by Gary Hurst. The circuit unlike the MK 1.5 had a 3 Transistor circuit which, based on production/supply comprised of either OC75’s or OC81D transistors. This OC81D Loaded MK II can be heard on many Jimmy/Led Zeppelin Tracks such as ‘Whole lotta love’. This was the pedal seen being used by Jimmy on the recent ‘It Might get loud ‘ documentary. Tonally the OC81D model has much more warmth and saturation than its OC75 counterpart.