The Atari 1200 and the finally Atari 1200XL have no real differences accept in the label and the casing. The label lacks the XL (eXtended Line) designation and the case is a smooth finish instead of the rough finish on the sold versions of the XL's. The only other interesting note is the fact that this system was modified, whether it was done in-house by Atari or done by someone outside of Atari along the lines of the Brent Borghese 1200XL OS modification are unknown. However this Atari 1200 has Atari 800 OS ROM's contained within it instead of the Atari 1200XL ROM's, also the 800 piggybacked ROM's have a handwritten neon green label stating 11/16/82 on them.
For many years I have been tracking down the elusive "vapor" trail of what was the earlier Atari 1200 design (sometimes referred to as the Atari Z800, then as the Sweet-16 and also as the Atari 1000 and 1000X line. Finally during some research on the Atari 1400XL for a friend and fellow Atari enthusiast I stumbled upon a lost treasure trove of materials pertaining to the very early designs of the Atari 1200XL.
I have spoken with numerous Atari engineers, Dave Sovey in particular had informed me of a side by side "Sister Project" of the 1200XL and that there were actually two versions in development. Several years later Dave's information had been verified when I found a copy of the Atari Sweet-16 Spec's, it was a very early document discussing an Atari 1200XL which much more expansion and capabilities then what eventually came onto dealer shelves in 1982.
The Atari prototypes came with an "X" designation, followed by a revision # and then followed by an A designation (whether A stood for Atari or for an Alpha version is unclear). As shown in the above Revision Schedule, designs started in 3/26/82 and went into 5/13/82. What's not shown is the date for revision X8A which was from other documents in early July 1982.
Interestingly enough, this above photo plot also from July 82 shows revision X10A which was nearly identical to what becomes the production Atari 1200XL motherboard. Of unusual note is the project appears to have not one, but TWO codenames, apparently this is where the original S-16 (Sweet-16) project takes a fork in its design, rev X8A also says Elizabeth on it, but in a different location... So far there is no revision X9A, it skips from X8A to X10A and then to final production.
This is the actual PCB layout of the revision X8A PCB for the Atari "Sweet-16". This is where things get interesting. Click on the above image to examine it more closely... Note that the layout is entirely different from the production 1200XL computers. Take particular note of the upper right corner of this PCB design - there is a 50 Pin header footprint and the traces all lead to the major IC's on the board which means this was the very first PBI (Parallel Bus Interface) which appears to have been meant as an internal device interface versus the later XL designs which brought the PBI directly out of the system to an Edge Connector.
Next thing to note is the Upper Center edge of the PCB, note the 2 DB-15 footprints versus an Atari SIO footprint. This directly correlates with the early Atari Sweet-16 spec's document detailing the inclusion of 2 15 pin connectors to replace the original SIO and would allow for peripherals to automatically power up when the system powered up.
The departure from the X1A-X8A design to suddenly a radically different X10A design that appears at the same timeframe in July 1982 means that there must've been a side by side "Sister Project" of the Atari 1000 series being built and it moved into place to become the production machine leading to a disaster product launch for a system with plenty of sexy looks, but nothing under the hood, so to speak.
I am currently investigating the possibility of having a PCB design firm that still works with physical Padmaster films to reproduce actual samples of the X8A revision motherboard, the real "Sweet-16" computer system to allow for further study and examination.