The disc makes a short appearance around the 15:45 timestamp as people in the video tour the archives.
Here's a crop of the disc in question:

Earlier this year I happened to get in touch with an ex-Microsoft employee that worked on Neptune, Millennium and Whistler.
He said that he most likely still owns the last build they got to take home, and after a few weeks of waiting a CD labeled Neptune 5111.6 12-99 indeed turned up.
Receiving the image was both cool but also slightly underwhelming. The OS files happened to align perfectly with the long leaked "5111.1." Turns out we owned .6 all along.
The .6 you see on the disc's label does not refer to the build number itself, but rather the variant. Microsoft produced a multitude of these with a different set of internal components, registry hives geared for different audiences (such as OEMs), and other similarly small alterations.
In my opinion the variant claim is backed by the existence of a commented entry for "NTC Selfhost" in the list of Startup programs in the leaked copy. This image also reaffirms the legitimacy of existing Neptune images, bar a few missing folders. Subsequently confirming that Neptune media was built by taking a chosen stable Windows 2000 build and overlaying updated Neptune components on top. The base build for 5111 is build 2128. Feel free to check this yourself by diffing the aforementioned build's images -- a lot of files will match.
Another point I'd like to touch on is 5111.6 being the final build. The infamous wAxWaReZ NFO actually alludes to Neptune already being a thing of the past by the time it leaked: He NUKES Neptune !!
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╔═══════════════════════════════════════════════════════════════════╗
║ ║
║ Windows Neptune build 5111 ║
║ ║
╠═══════════════════════════════════════════════════════════════════╣
║ ║
║ Format.....:.bin/.cue File Count....:14 @ 20MB ║
║ Supplier...:Unca Bill Cracker.......:who knows ║
║ Packaged...:wAxWaReZ Util Type.....:O/S ║
║ Date.......:12/??/99 Protection....:None ! ║
║ ║
║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║
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║ ║
║ What does Bill Gates do one week after he puts himself in ║
║ charge of all software development and makes Steve Balmer ║
║ CEO ? He NUKES Neptune !! Oh m'gawd !! What a BAD move ! ║
║ I've been using this Developers Release since I got my hands ║
║ on it back in December and it's wonderful ! OK ... so it has ║
║ one quirky bug in explorer that won't open pages in the ║
║ correct frame of a framed web page. Big deal. ║
║ The original setupreg.hiv file has been replaced so this thing ║
║ NEVER expires !! Originally it was a 444 day trial that reset ║
║ itself when you reinstalled. ║
║ Only system need to worry about is make sure you have enough ║
║ RAM. I started off with 64MB of SDRAM and quickly found out ║
║ I needed more. 96MB is bare minimum and 128MB is preferred. ║
║ If I had a laptop, this OS would be on it faster than you ║
║ could say "BOO !". ║
║ ║
║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║║
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║ ║
║ Installation Notes : ║
║ Unrar and burn to CD with yer favorite burning util. ║
║ Doesn't ask for a CD key and as previously noted, doesn't have ║
║ any time limit on it any more either. ║
║ ║
║ A definite keeper for collectors --- ║
║ ║
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Big thanks to the ex-MSFT guy for staying in touch and offering to upload a piece of history. I'll probably disappoint you a tiny bit by saying that he imaged it as an ISO and not superior MDF, but I figured I should be thankful for what I got. There's not much hidden data one could pull out of a CD like that, plus he's a pretty busy guy so I didn't want to drag stuff on.
Here's the link to the ISO: https://drive.google.com/open?id=1FIrup ... vGUtDPnHox_
Probably the best 5111.6 copy we'll ever get to see.
There may be more images of different builds of different OSes on the horizon, but it's not 100% certain for now.