It is a simple square pipe with bigger opening at one side.
It is meanth to easily mount the timing belt.
Start by cutting a piece of belt that has sufficient length (about twice stepperpully to other pully.. plus some cm's for the mounting).
First fix one end to the left of the X-carriage the normal way with belt through the hole and strap a tierap around.
Then guide the belt at left around the stepperpully... back underneath carrriage to the other pully and guide the remainder between right and carriage through this part.
Do it so that teh tight opening is to right.
Guide the rest of the belt through the mount opening at right side of carriage an fold it back so teeth of belt lock the movement. Then gently slide the printed part over the belt with folded remainder in as far as possible to the carriage.
It the tension of the belt is too low, just slide the part to the right until remailder is free again and do it again by pulling the remainder with some more force.. To make shure you could place some marks on the belt wih a pencil to see if you tensioned a tooth further.