Radio Altimeter aerial?
Could it be part of the Radio Altimeter (not Rad Alt) for Pressure Pattern Nav?
Pressure Pattern Flying, IIRC from conversations with JDW 'George' Washington and 'Slim' Rosier who both knew about these things was....
...long range Nav typically used by Qantas in the mid latitudes and low levels. Think Hobart to Auckland 8,000 - 12000ft. The difference between Departure and Destination QFE was plugged into a formula (other variables being distance, radio altitude and change of latitude, I think) to come up with a constant heading to fly.....the Connie would fly at a constant Radio altitude (thus the altitude would vary) and a single heading was flown to give the maximum tail wind component for the sector.