Hey Thomas,
the “Stiffy” is this.
Note that on X-50 rails (unlike on X-35 rails), the third “stiffy” rail is not in line with the two others, as to be seen here, so attaching a Z assembly on the front and one at the back of the X carriage as well is no more possible if you add the stiffy (it isn’t anyway if you add a drag chain there)
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