Did you know there are green, blue and black PS One's?
If you ever stumble across those in a thriftstore for a few bucks buy them!! They get sold between $2-3k on ebay!!
They are so called debugging stations. Their primary purpose is to read data from CD-R on which PlayStation games are burned. 
Blue ones were first...

...and later then came the green&black one which also were called Test kits.

Test kits are generally the same hardware as a retail kit but without restrictions such as region locks or media checks. 

So with such a test kit, you could just play any burned games with different region Codes.

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Test kits are differentiated from Dev kits by a few things. Usually Dev kits have full real-time debugging abilities and more memory and generally don't come in retail casing until later in the product's life.

Those units are rare, and considered a must have for Sony collectors. It's been produced in limited quantity.