please explain-
Derry:and now?I mean where the tin stops at the top?
Mr Lamb:Now and then. in wet weather. It doesn't signify.

so here the author is trying to throw light on the fact that despite of having a tin leg mr. lamb isnt taken aback by it.
instead he pays no heed to his injury...since his leg is amputated it sort of causes irritation where the tin stops.