Yes that is the distance between centres giving a gap of 42mm inside the bars. I measured mine at 40.6mm. The reason it is smaller is because the distance is set by stacking up several pieces that are oriented such that z-height is critical. The problem is that the height of a piece is quantised by the layer hight it was sliced with, so can be up to half a layer out. It doesn't seem to matter though because the bearings on one side of the carriage allow horizontal movement.
