The Macintosh IIx was introduced by Apple on September 19, 1988 as an incremental update of the original Macintosh II model.
It replaced the Motorola 68020 CPU and 68881 FPU of the Macintosh-II with a 16 MHz 68030 CPU and 68882 FPU (running at the same clock speed). It also replaced the 800 KB floppy drive with the 1.44 MB SuperDrive (in fact, it was the first Mac to have one).
The Macintosh IIx was discontinued on October 15, 1990.
Reference: http://en.wikipedia.org/wiki/Macintosh_IIx