@koalar that was my understanding too.
I've got a growing pile of working rear dampers with knackered top bushes, which will probably end up in the skip.
Been looking for individual replacement bushes with the voids, but struggling to find them at a reasonable price. Currently I can get complete dampers for about the same price as 2 bushes.
The good thing about poly bushes is that they handle small rotational loads without failing because the inner "tube" is not fixed to the bush.
(Oh... and they last a looooooong time)