There will be some old grey hairs who cry, "being back the sextant", and the reply will be, "who's that? A sexy relative?"
Yes. A young student asked me recently: "wot's a sextant ? "
One could weep.
Question ? If you only have 2 FMC's, and they disagree, how do you know which one is inaccurate ?
"In My Day" we had 3 INS's - and a standby Artificial Horizon as well.
( but then we also had a Radio Officer, a Navigator, and a Flight Engineer - Ahh! - as well. Just sayin' )
Flying into Moscow we had to convert our "Imperial" altimeter by way of a graph table to arrive at the various correct "metric" altitudes that we were given by ATC on approach, or departure. We asked Management if we could have a metric altimeter fitted, just to make life a bit easier. No, was the answer, we'd have to fit two to cater for redundancy, and then we would have to fit a third to cater for the anomaly between two differing altimeters. Can't afford it.
I guess it is all different today ?