1.5 may not be the correct term for this pedal but it's the one we all use so lets go with it. These weren't just made between the 1 and the 2 how ever they are very limited and were not part of the main range. The biggest difference of the 1.5 is unlike the 1 and the 2 which have three transistors this has only two which makes it much more like a fuzz face circuit than a traditional Tonebender.
As a result these fetch a very high price on the pedal market and are the subject of many Tonebender clones.