Landroger - the 'tube' houses various nav antennae - can't remember offhand which - it's less obvious than it appears in this pic which is at just the right angle to accentuate it.
It was always impressive how similar the ADCs' outputs were compared to the 747 of the same era, and M2Dude has mentioned how the RVSM trials showed their accuracy.
Of course there was always the infamous OAF 'glitch' which threw up false ADS warnings accelerating through M1 which happened regularly during my time on the fleet and was subject to a tech log supplement. It never seemed to affect the machine in any other way. I dunno if she did this from new or it was a result of her nosejob.