Atari 7800:Atari 7800 Model Differences
Page content & researching collaboratively taken from io55.net's Atari 7800 page with permission.
There are two 7800 models, and two revisions within one of the models.
Models and Revisions
Models are arranged chronologically. Revisions are underneath their respective model.
7800 with expansion port (1984/1986-1987)
The original run of the 7800 from 1984, which had to cease production during the year due to contractual disputes and a change in corporate ownership. These consoles were produced at Atari factories in El Paso, Texas and Taiwan, though none were sold until the 7800's launch in May 1986, although this is disputed, as some claim they were sold in Southern California during June 1984 (needs research!) The only noteworthy difference to later runs is the presence of an unused expansion port on the left side, originally intended for a planned LaserDisc add-on.
7800 without expansion port (1987-1992)
Identical to the 1984-manufactured 7800s, except the expansion port is now gone.
Expansion port indent (1987)
Outside of a minor motherboard change, this version of the console has no expansion port, with only an indent located where the port used to be. This version may have a compatibility issue with the 2600 version of “Dark Chambers".
No indent (1987-1992)
Same as above, except either a small outline or nothing at all replaces the expansion port indent. The aforementioned “Dark Chambers” defect may not exist; however, games using FE-bank switching may be incompatible instead (Centipede, Kool-Aid Man, etc). Most of these variants were manufactured in Taiwan with later units being manufactured in China.
All PAL 7800s are of this variant, as the console was not launched in PAL regions until 1989. PAL 7800s also feature a different faceplate than NTSC models which looks similar to that used on most 2600 Jr. consoles and have a green power LED instead of red.
References
https://en.wikipedia.org/wiki/Atari_7800#System_revisions
http://www.ataricompendium.com/game_library/easter_eggs/7800/78prosystem.html