Search as I might, I couldn't find them labeled on any schematic or mentioned in any fact sheet.
My guess is Shuttle bay interface/attachment, but I also could not corroborate it by searching engineering journals from the 80s and 90s.
It may also have been discolored after a while, or painted yellow (not necessarily actual paint; they could also have been replaced) during one of the service missions (again could not find anything). My understanding is that yellow is used for the parts the astronauts can grab onto.
Photos below (click to expand) show the structures, and the original color(?) in the Shuttle bay before launch.

Image sources: NASA, Public domain, via Wikimedia Commons; NASA, Public domain, via Wikimedia Commons



