Product review (1 stars): Beware of crappy filters being sold under the same HAC-504 part number!
As so many other commentators have posted, the original filter thickness was 1.25 and now much of what is being offered at Amazon instead ships thinner filters (0.75) which are GROSSLY  inferior  See the difference at [ ... ]g and http://www.amazon.com/review/R3IT1HZIKZX5WY?ie=UTF8&videoPreplay=1 .  Moreover, while some of the photos shown would (proportionally) appear to be the original thickness what Ive received from several retailers are the thinner filters.

For a great discussion thread on this topic and instructions on how to buy the original filters (HAC-504OS, $12.99 ea) see http://www.amazon.com/review/R21QW3XX459H67/ref=cm_cr_pr_cmt?ie=UTF8&ASIN=B000RDAORO&linkCode=&nodeID=&tag=#wasThisHelpful (scroll up to see the original comment by CTB.)