If you look at the ISO url, you'll notice something that looks like a short Git SHA hash. That's the exact git checkout of `nixpkgs` used to build the ISO. All system-level artifacts built with nixpkgs wind up with it as part of their name. Very handy.
If you check both, you'll see they're both commits from the upstream `nixpkgs` repository, just a couple weeks apart. So it looks like your torrent is likely an old official ISO that could be upgraded in place the same way the newer ISO would need. (Note, I haven't confirmed that, someone could've uploaded a malicious file with a nice filename, etc. Don't take my word for it, etc.)
(edit: ah, and if I had to guess, the 776 and 1023 as part of the filename is probably the number of commits since the original 18.09 tag was created.)
0, https://linuxtracker.org/index.php?page=torrent-details&id=9...