Sony's Official PlayStation 1 PRO

[FTLOTG]

This isn't well known information, but the PlayStation had a revision starting with SCPH-5000 consoles that improved the visuals of games. The original hardware revisions had simpler shading resulting in banding. When you read publications comparing early PlayStation and Saturn multiplatform titles it's worth keeping this in mind - visually they were much closer with Saturn having an edge in shading until 1997! The reimplemented GPU in 5K and newer models introduces more steps to shading allowing for a smoother gradient, this impacts all 3D shaded scenes.

(Taken from PSXDev)
It's even more apparent when seen in action:

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While not all SCPH-5000 consoles had the improved GPU, all 5500 models (and newer) did. The 5500 models may be the most desirable as they retain the original 1000 model audio DAC and the improved shading.

 

[Paweł]

wow that is pretty crazy, you learn something everyday.

 

[sama_lton]

And it didn't even cost people an extra $200 for the improved performance, that's CRAZY!!

 

[Viviana Santos]

Wow, I didn't know that, that's really cool.

 

[Frickinfrik]

Huh, that’s pretty cool. I never knew that. The more you know.