The IBM PC AT was IBM's second generation PC (if you consider the XT an extension of the original PC). It was introduced in 1984 and included several "new" features such as a 6 MHz Intel 80286 processor, a 1.2MB 5.25" floppy drive and an improved keyboard. The typical AT sold with a 20 or 30 MB hard drive and 512K of RAM.
The IBM AT in my collection is one of the most pristine ones I've ever seen. It is immaculate inside and out and even has the original Computer Memories Inc. hard drive (there are very few of these that didn't fail early in their lives.)
The rear of the unit shows the slot ports as well as the lack of a cassette port.
With the trim plate in place the rear view is cleaned up considerably.
IBM took an oddball route to adding RAM to the AT. To conserve money and motherboard space they piggybacked RAM chips on the 512K version of the board.
In addition, this AT has all of the original documentation, the original plastic "trim plate" that cleaned up the appearance of the back of the machine and even the original IBM logo keys to lock and unlock the case and keyboard.
The machine also came with an early version of Microsoft Word and a very early Microsoft Mouse.
This system was donated to the Computer History Museum and is now used for educational demos.