Looking at the assembly photo #2 Z Axis, is the power supply to the rear of the photo? In other words, do the Z axis motor wires exit toward the power supply, or away from the power supply?
This is important because it also determines which way the end stop holder face, and it determines which side the 5mm spacer goes on.
I assume that the Z axis motor wires and the end stop holders face the same way on both sides of the printer.
Since there is only one 5mm spacer, but the screws on both sides are the same length, what happens on the other side? Without the spacer, the screw appears to be too long.