Series numbers go from 21171 to 21173, skipping 21172 - which should be for block 275087. I've only been mining with Slush for 2 weeks now, and I have no reason to assume that the block won't show up eventually, but hoping some of the more seasoned miners in the pool can let us know if they've seen this happen before and what we might expect. After what happened with block 21027 I'm admittedly a little nervous though.