I'm not sure, but the part looks like the guide for the timing strip. It is located deep inside the print head carriage, in the red marking:
I wonder why you disassembled the printer to that level? In my case I did this to look for burnt parts after having installed a print head that was not completely dry before installation. That printer of mine is only kept for mechanical spares.
The optical reader for the the timing strip might work erratically or not at all, if the guide is missing. It holds the timing strip at the proper position relative to the optical reader. The purpose of the timing strip and the optical reader is to determine the horizontal position of the print head carriage. The printer will print erratically or not at all, if the optical reader malfunctions.
It is also important that the timing strip is positioned correctly, when it is put back in place. At one end the timing strip has an "L" marking. It is actually the letter "L" for left, and it must be placed at the left side of the timing strip and read as a proper "L" when viewed from front.