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Thanks for posting. It looks to me you have either our 39-253RB & 39-253LB, or the 39-438B & 39-439B operators. These two sets of operators are nearly identical save for one key difference: the appendage at the end of the straight arm. The 39-253RB & 39-253LB use the square shaped 39-529 Track Slider sold separately, whereas the 39-438B and 39-439B use a rectangular slide that comes with the mechanism. See the video below for reference. Remove your old mechanism to determine which you need.