Maybe someone with more 360 experience can help with this - I have a 360 that has no HDMI and the square pin in the middle of the power connector. The PSU is 203W. This sounds like it should be a Xenon - but the rear panel is marked "Service Date: 2009-07-24" - which I would have expected to mean the main board had been replaced by an Opus. Is is possible that it was sent in for service but it still has the original mobo in it?
Yes, it is possible. Opus motherboards were only given out when they have run out of Xenon stock. Your Xbox must have been sent to a place where they still had replacement Xenon motherboards. I have seen a few Xenons from 2009 myself.
OK, it is a Xenon - I wouldn't have thought they would still be using those boards in mid 2009, but apparently they were. It does have the Zephyr style additional GPU heatsink on it, though. Looking at the board, it appears to have been refurbed - some of the parts have much older date codes and there is sticker residue from the old barcode sticker. But with that date I assume it's got the new JTAG resistant bootloader...
None if these other identifiers matter or are conclusive, post a picture of the power connector that is on the board.
