Yes, indeed, the black object is a darning egg. I have two of them, one from my mother and one from further back. Mine are not decorated and were well used. I used to have to darn my fathers socks with that soft multiple ply thread you referenced. When I first started quilting I used that thread to baste with. But, the thing that strikes me about your box is the fact that it transcends so much time. Goes from button hooks to modern thread. Look at the labels on the thread-Coats and Clark duel duty thread. What a fun thing to have. I remember my grandmother used one of those curling irons she heated on the stove-her salt and pepper gray hair was often yellow-because she scorched it.