Thanks Greybeard, they certainly look like provisions for a third Rad Alt – same size, same distance apart & the wedges give them the same vertical directionality.
The question now is why were they put there? I was not aware of the requirement for a third Rad Alt for triplex autoland but the 737 only has 2 autopilots. The latest versions are Cat IIIb capable with a rudder channel (see other threads) but I am sure a third Rad Alt is not required, it doesn’t appear in the MMEL. Can anyone operating the 737 to Cat IIIb confirm this?