No definitely no third Rad Alt - and it has never had one, we have had the aircraft from new. No signs of a gap or alteration in the in the c/b’s either.
The 737 A/P’s are dual-dual and we only have 2 FCC’s, IRS’s, ILS receivers and Rad Alts. I don’t see where a third would fit into the system, unless it was a backup for Rad Alt #1.
Have any other 737NG operators noticed if they have these blank extra Rad Alt aerials? They could have been standard fit in anticipation of a development or other reason that has since become redundant.