Pardon the bad late-night, hand-held, via iPhone close-up photographs. And yes, it was indeed corrosion.
A friend's machine that he simply set aside over 10 years ago when he thought his HD had failed. The circuit path from that "missing pin" travels up to what appears to be (only), a probe test point. I hope to get back to this project today and I've advised him of suitable replacement boards (Apple 820-1173-A), available on eBay.
And yes, the machine looked like it was stored outside with the case open, possibly in a barn. I should have photographed a series of "before images" to provide under the new topic of "How Not to Treat Your Mac" to possibly send many here shrieking from their own machines in definite horror. (Feeling a bit like Vincent Price there, shivering with delight, standing next to a working bug zapper.)