Atari later upgraded the basic design in 1986 with the 1040STF (also written STF). The machine was generally similar to the earlier 520ST, but moved the power supply and a double-sided floppy drive into the rear of the housing of the computer, as opposed to being external. This added to the size of the machine, but reduced cable clutter in the back. However, the joystick/mouse ports, formerly on the right side of the machine where the disk now sat, had to be moved to an awkward location in a cramped niche underneath the keyboard.
The 1040ST was the first personal computer shipped with a base RAM configuration of 1 MB. When the list price was reduced to $999 in the U.S. it appeared on the cover of BYTE in March 1986 as the first computer to break the $1000/megabyte price barrier. However, the ST remained generally the same internally over the majority of its several-year lifespan. The choice of model numbers was inherited from the model numbers of the XE series of the Atari 8-bit family of computers. A limited number of 1040STFs shipped with a single-sided floppy drive.
The same basic design was also used for a cut-down version, the 512 kB 520STFM, which replaced the earlier 520ST models in the market. The early 'STF' machines lacked the 'M' modulator that allowed a TV to be used and would only work with a monitor.
Source: Wikipedia, "Atari ST", available under the CC-BY-SA License.