Top part of block and tackle

This is an older block and tackle rod balance. I'm afraid that these aren't made anymore, though even when they were it was not possible to replace just the top piece by itself. These are all one whole assembly and removing the top plastic pieces makes the whole thing spring apart. That's why your balance has come undone like it has. 

Either way, we can still offer a modern channel-type balance that can replace this. Can you just show me the shoe or clip at the end of the cord? There may also be a "T" shaped shoe at the bottom of the balance, instead. Whatever it is at the bottom, we need to see it. Once we have, we can recommend a substitute.