2: Sony's first of its 4 entries into the video game world, released in Japan in December 1994, then entering the US and Europe in September 1995.
It began its life planned as a CD-ROM add-on to Nintendo's Super Nintendo Entertainment System (Super Famicom in Japan).
When that partnership fell through, Sony decided to take the add-on they were already working on and make a stand-alone home console (originally, the Playstation was designed to play both Sony's games and SNES carts. The Playstation as we know it today is actually the Playstation X, hence the early 'PSX' abbreviation).
Five main versions are known to exist:
Model SCPH-100X: This version was infamous for early versions being able to play imported and copied games with a simple disc-swap. A later version corrected this issue, but both were also known for overheating problems and for having streamed audio and video skip frequently.
Model SCPH-550X (I don't personally own a console with this number. Can anyone verify?): Fixed the overheating problem, partially. Skipping problems weren't addressed, however. Standard RCA cable output ports were replaced with a proprietary output port, according to sources. It's also claimed that the laser housing was moved.
Model 750X: Addressed the audio/video skipping problems. I can confirm the laser housing was moved and that the audio/video output port exists.
Model 900X: Final revision of the PSX hardware. Parallel I/O pot removed, as Sony didn't use it, but Datel's Game Shark and other unofficial peripherals did.
Sony PS1: An all-new console design that was supposed to allow the console to be travel-friendly, provided you bought the screen for it. Name changed to PS1 because of the upcoming Sony PS2.