If it's any help, here's a shot of it from the other side
I seem to remember being involved in a fair of bit of discussion about this on the SRG site. After referencing a couple of cutaways and looking at the nature of the damage, I came to the conclusion that it has been mounted upside down and is possibly the underside & rear edge of the starboard front spar fairing. That said, while a lot of people thought that quite plausable, I've never got my hands dirty on the old girl (not for lack of desire to) and don't know exactly what's been acounted for so I'll leave all further speculation to those who do.
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One by one, the penguins are stealing my sanity...