I am not 100% certain, but I remember that on my sigma 150mm, the very last element is sealed to the barrel one, as in it does not move with focusing. Lens in the 150mm is normal, even bnib in local shops will have dust. Out of the 3 copies I tried at a shop, 1 had some fungus, the other had an insane amount of dust and the last one which I bought, also had quite some dust for a new lens.