Warning: Spoilers spoken here.
Of course you’re all right. It’s the lock box.
Commemorated in this scene, which begins
It was entirely involuntary, a knee-jerk reaction. She didn’t stop to think about it, she just picked up the little tin lock box and let fly. It’s arc was swift and her aim was true. The box caught him just above the left eyebrow and burst open.
but also for what Kate finds in it before she launches it as an offensive weapon slash foreplay. It’s also a pretty good symbol, if I sez it who shouldn’t, of Kate breaking free of her grief over Jack to begin to live again. Even if it is with the single most unlikely guy in the Kate ‘verse. [Bonus points to anyone who remembers which book in the Kate series first signals Jim’s other than professional interest in her.]