What iMac model have I got?

Hi I am wondering as to what iMac model have I got?

My serial number stated in the 'About This Mac' location states that the serial number is:

W8******X85

I wish to know what model I have as I don't understand why it does not match up with the given serial numbers allocated to any of the aluminium bodied iMac pre-2009 serial numbers

Sorry for this dumb question but I wish to know what model iMAc I have as I am thinking of upgrading the CPU memory as I have only 1GB which ***** in terms of speed

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