plane from sides 16 and 18 are notched flanges designated generally 24 and 22 respectively depending inwardly from top 14 and bottom 20 are top offset flange 26 and bottom offset flange 28 which are offset on opposing sides of the plane defined by notched flanges 22 and 24 the advantages of offsetting these flanges will become apparent from the following description referring now in particular to fig2 each of the blades of the preferred embodiment damper comprises a hook shaped hinge portion designated generally 120 two blade faces 122 and 124 which are offset with respect to each other by the thickness of the notched flanges and two hook shaped end portions designated generally 126 and 128 extending longitudinally for the length of this blade as seen in fig2 each of these blades is uniform in its entire length except for a blade clearance notch designated generally 200 which defines a clearance edge 202 and an interference edge 204 which interact with the linkage rod of the damper as described hereinafter with the exception of the aforementioned notch 200 the preferred embodiment blade as illustrated in fig2 including the hook shaped hinge portion 120 and end hook shaped end portions 126 and 128 may be configured as described in my prior co pending patent application ser no 764 774 filed feb 2 1977 entitled 34 rotating blade fire damper 34 which has been incorporated by reference herein referring now in particular to.