Originally Posted by
Graybeard
The manufactures have sometimes leased back a new airplane for flight testing, and they could have added the third radalt. If so, they must have decided to leave the antenna mounts in place, rather than remove them and plug the holes.
Slight clarification: the Cat IIIa autoland, such as the MD-80 and 737 are simply dual, not dual-dual. They don't have the full redundant circuitry within each Fright Guidance Computer, as they are fail passive. If there is a failure or mis-compare, they give the plane back to the pilot. A Cat IIIb autoland will continue the approach and landing after a single failure, which requires dual-dual circuitry. Note, that does not mean 4 sensors each, but just two. The Triplex autoland uses 3 relatively simple Flight Control Computers and 3 sensors each.
There was no point making the 737 dual-dual or triplex, until it had a fail operative rudder.
GB
But the newest 737's ARE Dual-Dual, and are certified to Cat IIIb standards.
AFAIK most 737NG aircraft post c.2004 are kitted out for CAT IIIb, to enable them to compete more effectively with the Dual-Dual A320s.