Way back in November of 1964, IBM announced the smallest member of the System/360 family, the Model 20. According to Wikipedia, The Model 20 supports only a subset of the System/360 instruction set, with binary numbers limited to 16 bits and no floating point.[1] In later years, it would have been classified as a 16-bit minicomputer rather than a mainframe, but the term “minicomputer” was not current, and in any case, IBM wanted to emphasize the compatibility of the Model 20 rather than its differences from the rest of the System/360 line. It does, however, have the full System/360 decimal instruction set, that allows for addition, subtraction, product, and dividend of up to 31 decimal digits.
By the end of 1970, it is estimated that more than 7,400 Model 20 processors were installed in the United States. Over time, the Model 20s were replaced with faster and more powerful models.
Click here for an article that chronicles the rescue and resurrection of a Model 20 in Nuremberg Germany. As an added bonus, the team may also have discovered a System 370 Model 125 originally announced in 1972.
When someone says that the mainframe is dead, I think we can be confident in saying not only is it alive, but perhaps even a Phoenix that lives again.