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Julian,
If I remember correctly, this is the area below the heavy center section channel where the landing gear fits. If the rivet shop heads protrude into this area, you will not be able to get the landing gear legs in there. ALL RIVETS on both side of that channel where the landing gear attaches should have the factory head towards the landing gear. It might not be necessary to have every rivet inside of the channel orientated that way, but by the time you figure out where you can reverse them without causing problems later, you could have already completed the job the hard way. Sometimes it is easier to go ahead and do it the hard way.
Joe