During the rebuild, I saw the organ builders playing with what looked like square beads for threading onto a child’s necklace. That or they looked like the corks jackaroos are supposed hang from their hats to discourage flying insects from swarming.
In reality they were making little supports for the tracker action. Going back from the manuals to the rollers is a long set of trackers that are horizontal. Because these cedar trackers aren’t weightless (nothing is, after all), and are so long, they will inevitably sag in the middle. The concern is that pressing the key will then only pull the tracker strips taught, failing to actually move the roller at the far end from the key. The result would therefore be a failure of the instrument to respond to pressing the key. I suppose the tracker could be pre-tightened to reduce the degree to which the strip sags, but then pressing the key would need far more force to be exerted by the organist. Some gearing adjustments might help – but the knock on might impact on space or responsiveness or the distance the key needs to be depressed.
So the blocks on strings are glued to the trackers and hung from a bar above the trackers to give them extra support, but allow freedom of movement, allowing the tracker to move back and forth.