The metal rails are permanently secured to the MDF base, which is (I think) either 12 or 18 inches wide by 6 feet long. If you look at the end of the pink arrows, you can barely see the fender washers that hold the fuselage stations to the rails by gripping under the little lips on the rails. By loosening the wing nuts you can slide the stations along the rails, then secure them by tightening the wing nuts.
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One modification I would suggest is to add pieces of wood to the bottom of the station that are perpendicular and flush with the rails. One of the big pains with this jig is having to make sure that the staitons are square and centered each time you move them.