That y-cable assembly was called the "centerline camera crosshair". It worked in conjunction with the docking camera below it and the docking target on the ISS.
The crew would use the centerline camera to view the relative alignment of the crosshair and the docking target during the final stage of rendezvous/docking.
The best drawings I could find with the crosshair labeled:
From Space Shuttle Systems Handbook Volume 3, pages 389 and 391 in the pdf.
There is some video of the centerline camera crosshairs at 4:18 in this STS-86 postflight video (in this case the station shown is the Mir).