Part of the myriad of huge amplifiers Yamaha produced during the 1980s : well-built, powerful enough to almost drive a pair of Magnepans, looking in sync with the times (flashing red and black) and nearly indestructible.
Auto-Class A was for the later M-80 revision but the ZDR system was applied here as well as a fully DC circuit. ZDR (aka Zero Distortion Rule) is a way to re-inject the detected distortion signal but in phase-inverted mode.
The M-70 was a B-70 in Japan and a Stereo Sound "Best Product" there, alongwith the C-70.
The smaller M-50 was introduced alongside ; later replaced by the M-80 / M-60 / M-40 in 1984, all three with the added "Auto Class A" feature, somewhat bigger looks plus even more available power.