Bad photos but here are 2 PS2's I came across in a show.
Left one is the first PS2 prototype. Had huge overheating issues so the right PS2 prototype was built with overkill cooling
There are also a few discs of Super Mario floating around that will run on Playstations that were made for testing this thing, super rare.
anon that looks like a PS2 slim, yes early ones of it had huge overheating problems. The one on the right is the TOOL PS2, used for debugging games, but I am not sure exactly if it was a full dev kit or just a test machine.
Better image:
This is the Slim, although the one you posted has a shiny side, looks like a prototype all right. (edit) it came out long after the original PS2, it certainly was not a prototype for the original, but may well have been a prototype for the slims, A friend had a production slim. Always amazed me that they could make it that small. But as stated above they had over heating issues at launch, they were fixed eventually.
http://www.hotgamez.com.br/imagens/hotgamez.com.br/produtos/CONSOLES/PS2/PS2_2.jpg
I own a production slim and while I am only going by what the guy told me at the time, This thing was much smaller than a slim. It was a lot denser, heavier than a slim and about 2 thirds the size.
The guy who owned it specifically said that the smaller one was a predecessor to the larger one which was developed into a dev kit (I think your right on the second one there) but the idea was to build a console as small as possible following what they did with the Play Station and Play Station one.
In all honestly I dont really know a lot about this console as I only owned a PS1 up until 3 years ago when I bought my slim in a used clothes store for $40 AUD
I was just judging from the size of the controller to the console.
And that discs are 12cm across so the size of the slim is largely dictated by that. The one in you picture looks to be the exact same size as the production ones. I suppose they could have been working on it since conception but it seems unlikely they would have one this far along at the same times as the original one what with all the problems the production slim had at release.
The originals also had a PCMCIA port as standard and IDE connections for the internal HDD and modem expansions, versions of the originals dropped the PCMCIA port early on, and the later versions of the slim did not even have solder points for the IDE HDD (early versions had the points but nothing attached). It would be strange to develop a version of the console early on to not include most of the original features and then add them, much more likely to remove feature as the console evolves to cut more costs. The exception being the IR receiver.
The Slim also had an IR receiver which the original did not, barring a few of the special editions that came out in colours other than black around the same time the slim was releasing. It would seems strange to include the IR on the slim and then drop it for the original one especially considering that they released a remote for the original anyway for DVD that needed their own adapter, would have been easier to integrate it from day one rather than add it back in later.
The buttons are in the same place and labelled, it is the same thickness just eyeballing it and all the ports are in the right place to be a production one but like it is, it has a shiny side which production never did, that is just what lead me to think it might be a early version of the slim but not the original.
There is also the cost thing, Sony loved to price down every part of the PS2. Case in point the drive, they may all look the same sliding in and out but as the years went on whoever sold the cheapest drive got the contract. You can get PS2s (the big ones) with drives from Sony, Philips, and a few others not that I can find the info now. So cost cutting was always high on their list, making a smaller one would have been more expensive so I imagine this was later life after they had made a decent profit to be able to mess around with slim ideas.
On the tool PS2, they were massive for all the extra hardware and no doubt cooling too but not solely for the cooling, It had S-Video out, Ethernet port as standard, expansion slots for extra cards (frequently more ethernet ports), VGA port, etc.. They also had an external HDD cage as opposed to the internal one in the production models. It was bigger to incorporate all the extra hardware the dev kit needed, not some knee jerk response to the slim having cooling issues, this was around before the originals too which would explain the bigger size a little too, things tend to get smaller in the electronics world with age rarely the other way round.
(That also looks like a full size ATX power supply, the socket and switch are in the right place and it has a fan in the PSU, there is also a serial port too)
This is the external HDD cage for the dev TOOL PS2, fun side note the one for sale is running Red Hat 6.2, not sure if that is normal but fun.
Not saying the guy was wrong, just seems unlikely that it would have been a slim prototype from before the original all things considered.