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You may have already guessed... these are hot tracker strips in the PT (periodic trigger) events. I believe the most recent overlay events are selected over the full precession period (~55 days) and the selected period started close to the beginning of science ops. See this presentation from Hiro on Tracker Status at the Sept September 2008 collaboration meeting. The two layers outside specification at that time were Tower 0 X7 (32 strips) and Tower 8 X17.
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