Why did Mary pack a clock?

In the episode where they all cross the border and make a new friend / future husband — why did Mary pack a whole clock? I don’t think it was ever fully explained.

Was it a family heirloom and they were at risk of burglary/looting? That’s the only possible explanation I can think of. But if that’s true, then they wouldn’t have been discussing which of the two clocks to take (unless they were both heirlooms and they only had room for one) … help me out, I’m confused!