The mast on which the Unified S-Band high gain
antenna is mounted.
Note that the antenna is mounted approximately 180° from its proper mounting position
(because the antenna wouldn't be visible in the display if it were mounted in
the correct location).
There are relatively few in-flight photos of the USB antenna, but ones I did
find (e.g., AS15-88-11961
do not show the Kapton insulation depicted here. Pre-flight photos (e.g., S69-19197
also lack the insulation, but it's hard to say whether insulation might have
been added after those pre-flight photos were taken.