A few questions for you :)

[PieEater]

There is a section of the stub which is solely rubber but as others have said, it's pretty thick.  I've never seen one crack all the way through but I did find some this year which had started to disintegrate and crumble.  They were from the same bike and it's not something I've seen previously.

Thanks for the confirmation Mike I have updated the FAQ - "The rubber on the inlet manifolds between the carbs and the cylinder head is prone to cracking, this is generally cosmetic with no reported issues of the cracks affecting the internal walls of the manifolds or the performance of the engine."