The name "Tone Bender" was used on a remarkable number of fuzz pedals over the years. The original pedals were released under the "Sola Sound" brand, which was a house brand for the Musical Exchange in London, now called Macari's.
Sola Sound Tone Bender (Mk I)
The retroactively named "Mk I" was the original Tone Bender. A short run (perhaps less than 100 by the maker's account) of wooden boxed models started in April 1965. By the end of the year, a folded sheet metal version was available.
The circuit is a modified Maestro Fuzz Tone FZ-1. The modifications are credited to Gary Hurst.
Sola Sound Tone Bender (Mk I.V)
Also named retroactively, the "Mk I.V" was simply labelled "Tone Bender" but had a new cast metal enclosure and appeared in 1966.
The circuit is a 2 Transistor English Fuzz.
There is a controversy as to whether Gary Hurst, Dick Denney, or someone unknown designed this circuit.
Sola Sound Tone Bender Mk II Professional
Released sometime in 1966, this was the first actual use of the "Mk" moniker on the pedal's graphics. The enclosure was also cast metal, similar but not exactly the same as the earlier "Mk I.V."
The first transistor stage is a No Bias circuit, similar to the 3rd stage of the FZ-1. Following the first stage is a 2 Transistor English Fuzz.
The version was famously used on the early Led Zeppelin albums by Jimmy Page, and is generally considered to be the definitive Tone Bender.
The design of the Mk II is credited to Gary Hurst. This was Hurst's last known Tone Bender design. He would reappear later with a line under CBS/Arbiter circa 1970s, and again with a new JMI company making fuzz pedal reissues circa '90s/'00s.
This pedal was distributed world wide starting in 1967 and is perhaps the best known version of the Tone Bender.
The circuit is a 2 Transistor English Fuzz, like the "Mk I.V" and Vox Distortion Booster.
Designer is controversial, as in the "Mk I.V" entry.
Sola Sound Tone Bender Mk III
The Mk III appears in 1968, and the Sola Sound branded version is rare. The Vox branded version is more common, and other brands (Park, Rotosound) are seen as well.
The circuit has a Darlington pair common emitter voltage amplifier that leads into a No Bias (diode variation) fuzz circuit followed by a passive filter network with variable "TONE" that mixes between two filter paths.
Designer is unknown, but the No Bias (diode variation) may be clue as this is found on a number of contemporary units including the UK Baldwin-Burns Buzzaround and several others.
Sola Sound Tone Bender MK IV
The Mk IV is either identical to or very similar to the Mk III.
Released around 1970.
Colorsound Supa Tone Bender
The Colorsound brand was introduced around 1970 by Sola Sound. In 1973 the Supa Tone Bender appeared.
The circuit is a copy of an Electro Harmonix Big Muff Pi. Some components are left out, notably the 2nd stage clipping didoes.
Jumbo Tone Bender
The Jumbo Tone Bender came in 1975.
The circuit is a simplified Electro Harmonix Big Muff Pi with only 3 stages. It is similar to the Supa, but with the final make-up gain (post tone circuit) amplifier deleted.