Well, the crabs will pretty much try to carry off anything they can that has any kind of food value - tin foil, wrappers, cardboard food boxes, etc, but I don't think they have much interest in a sextant box by itself unless there was some residual food still attached. I don't see crabs moving a sextant box very far, and a bigger box would be very difficult for them to move. When everything dries out, they'd loose interest.
The Gigdies are a conundrum. On one hand they seem to be well crafted, which does not speak castaway to me. On the other hand, we can't figure out what they were used for, so could be related.
Personally, I think they were used to hold screening in place on a wood frame, with an action where the teeth are rotated into place to hold the screen. That says colony to me, and I'm imagining the island carpenter making a screens for "Gallagher's House" at the 7 site, and using some aluminum he found to make the screen holding gidgies.
Then there are the US made wood screws, so that shifts my thoughts to screens at the Coast Guard station.
Andrew