Dolby Stereo 70 mm Six Track refers to the use of Dolby noise reduction on the six magnetic soundtracks of a 70 mm print. This was first used on some prints of the MGM film Logan's Run released in 1976.
The Todd-AO format was introduced in 1955 and included multi-channel magnetic sound from the start, it does not have an optical soundtrack (although in recent years some 70mm prints have used a DTS digital track in place of the analog magnetic one).
The original layout was for 5 front channels and one surround. But by the 1970s the use of the intermediate (left-center and right-center) tracks had been largely abandoned, these channels either being left blank or filled with a simple mix of the adjacent channels. Dolby did not approve of this later practice, which results in loss of separation, but instead used these channels for LFE (low-frequency enhancement) utilizing the bass units of the otherwise redundant intermediate front speakers. Later the unused HF capacity of these channels was used to provide for stereo surround in place of the mono surround of the Todd-AO layout giving the modern 5.1 channel allocation retained today by Dolby Digital.