The gods of rock shined upon us as it is now clearly labeled the Effects Pedal port, as seen above. Opposite the page is an explanation of all the buttons and gadgets found on the guitar, including an explanation of the Effects Pedal port. How dost thou rocketh mighty ports of pedals?
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Rockin'! Now that we know for sure that it's for some sort of effects pedal, we curl up into the fetal position. We've been scorned in the past by these sorts of shenanigans. We're looking at you, Konami! We still want our Beatmania III pedal!
(Update - Because everyone is insane and paranoid about April Fools, we've included some actual pictures of the manual in question behind the cut. We even threw in some Mudkip because we heard that you like him so much.)
Mudkip is happy about Guitar Hero 2 for Xbox 360. Why can't you be that happy and trusting? Oh, right. The instruction booklet with the pedal blurb is actually an insert that comes with a sticker pack for your gee-tar. Don't worry manual nerds. You'll still get your full-color goodness.
Here is an actual picture of what we scanned at the top of the page. Feel free to toss it into Photoshop and zoom in all you want. Or if you're cool, you can hold down CTRL and use your mouse wheel, right?
Finally, here is the other side of the page that we quoted. Now you understand why we didn't just scan it. If you'll excuse us, we need to get back to unlocking songs.