I think all the Waterstones slipcase editions have the short story missing due to an error/oversight and it would be too costly to have them reprinted, so they are recalling them to put in some extra's the stamp and free Discworld cut out book.
I'm thinking there would have been the story included, else why all the blank pages which would just add to the printing costs, if they reprinted them they might not have all sold and leading to a loss of revenue on both editions.
Making their limited edition not so limited, so I would suggest go on to the website get the number to phone if you want your slipcase editions upgraded.