Most likly it is a motherboard that id made for HP by some manufacturer as a special order. These companies like HP and Dell build so many computers and therefore can order components and parts in such large numbers that they can get spacial made parts just for thier computers that are not sold to the public. So they can give a MB design or have the MB manufacturer design one for them and they will then order thousands of these custom boards. So a lot of times your going to open up the case of one of these computers and see a MB that you wont recognize or be able to get anywhere.
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If you have the computer already, the easiest way to find that out (I found nothing useful in searches) is to run a program called Speccy available here http://www.piriform.com/speccy/download
The free version works just fine - it'll give you all you need to know about your system hardware.
Hope it helps
go download a copy of SIW - run it and look under Motherboard it will tell you what it is - well a product designation then google it... SIW is good to find all sorts of details about the PC - including lost passwords!