The iTunes P5 RPU was converted to P8, L5 was set, L6 metadata was adjusted to match the UHD BD, and it was injected into the HDR BL. The iTunes HDR10+ metadata could not be injected due to a grade mismatch. 56 metadata frames were duplicated at the beginning and 54 were duplicated at the end to sync with the UHD.

iTunes vs. UHD grade check:
https://slow.pics/c/mnkhae2S
(with HDR metadata)
https://slow.pics/c/pSF1tdrZ
(without HDR metadata)
HDR plots: https://slow.pics/c/HbxUWtoA

Original remux notes

REPACK: Superior quality unrestored mono audio track.

Previous remux featured unrestored audio from an analog LaserDisc, which came with noticable distortion in louder parts. A VHS Hi-Fi release has been found to include the same unrestored mastering but without distortion, and I’ve synced it to the UHD.

In comparison, the UHD Blu-ray mastering renders a lot of music and sound effect cues lifeless. Both are included in this remux, but personally I see no reason not to go with the unrestored track anymore.

Unfortunately later releases, particularly digital LaserDiscs, all use degraded tracks. Still, VHS Hi-Fi is a technical upgrade over analog LD.

Audio comparisons:
https://mega.nz/folder/6MAy2KjI#Oukjc3JxpaQsJXxiFHxMfg
(Samples)
https://slow.pics/c/TallIgi6
(Spectrogram)
https://slow.pics/c/FBM5TlLi
(Spectrum analyzer)
https://blah-ray.blogspot.com/search/label/North%20by%20Northwest%20%281959%29
(Blah-ray write-up)

Thanks to captainsolo for providing the VHS track, and Moshrom for his extensive comparison of all the different editions.