BFI 4K Blu-ray

BFI 4K Blu-ray has excellent master and grade, no DNR and tasteful HDR

Restored original mono mix: Toho Japan 4K Blu-ray
Unrestored original mono: 2014 BFI Blu-ray

BFI 4K Blu-ray has OG Perspecta (in 5.1 container) and a stereo downmix of it; no original mono.

The unrestored track on BFI's earlier release appears to be an almost entirely unmanipulated transfer of the original mix. It suffers from a distracting low-frequency hum. The 4K restoration comes with a restored track, which appears to be the same transfer, but which appears to be cleaned up with a moderate amount of noise reduction and some EQ, nicely opening up the high-end, and which is likely to be preferable to most listeners. All earlier releases sound poor.

Perspecta isn't a discrete surround format, instead only relying on manipulating volume and panning of the mono track across L/C/R channels, based on embedded control tones. As such, even the Perspecta track is technically just the mono mix, though the omission of the proper mono on BFI's 4K is still unfortunate.