Hey David,
more than one user wondered when there came a “stiffy” third rail for the 50 mm X axis, because X-50 rails were introduced with “Rigidity is not an issue with 50mm Linear hollow shafts standard on the X Axis (Stiffy is unnecessary)”, but with Elite Series a “stiffy” 3rd rail was introduced for the 50 mm X axis. Here’s the manufacturer’s statement on this question.