In a linac-based X-ray free electron laser (FEL) there is often a need for energy chirp control of the beam before it enters the undulator. In Ref. [1] a corrugated, metallic beam pipe in round geometry is suggested and analyzed as a device that can be used to passively "dechirp" the beam. Compared to round geometry, at geometry - having the beam pass between two at, corrugated plates - has the advantage of allowing the dechirper strength to be adjustable by changing the separation of the plates. And, indeed, such a device was recently tested in a proof-of-principle experiment at the PAL injector test facility in Pohang, Republic of Korea [2], and another at BNL's ATF [3]. One disadvantage of at geometry is that, in addition to the usual dipole wakefield that is excited when the beam passes through the structure off axis, there is also a quadrupole wake excited, even when structure and beam are perfectly aligned. Show Partial Abstract