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From Wikipedia: http://en.wikipedia.org/wiki/Xbox_360

Xbox 360 Pro / Xbox 360 Premium

The Xbox 360 Pro[47] (sometimes referred to as Premium and packaged as simply Xbox 360 with the subheading "Go Pro") included all the features of the Xbox 360 Arcade and included a hybrid composite and component cable with optional optical out instead of a composite cable. This model also included a detachable Hard Disk Drive (initially 20 GB, while later models had 60 GB) to store downloaded content, provide compatibility with original Xbox games, and store game data. The included hard drive came with game demos, video clips and a free Live Arcade game, Hexic HD. In July 2007, this version of the Xbox 360 began appearing with the Zephyr motherboard (the motherboard used in the Elite) which features HDMI 1.2 output and an improved GPU heatsink. Although this package did include an HDMI output, it did not come with an HDMI cable.[48][49] Starting at the end of September 2007, the newest systems were shipped with the new "Falcon" motherboard. This motherboard includes the new 65-nm CPUs, making them quieter and cooler than the older systems.[50] On August 1, 2008, the 20 GB version was discontinued and was replaced by a 60 GB HDD model at the same price. Holiday 2008 consoles were bundled with Lego Indiana Jones: The Original Adventures and Kung Fu Panda.[51] Price cuts that took effect on September 4, 2008 reduced the price from $349 to $299.[52] The Xbox 360 Pro, following its discontinuation, retailed for $249.99 until stocks were exhausted.

From what you've said so far I'm guessing it's one of these. Did you get an HDMI cable with it? What's the manufacture date on the back?