It certainly makes sense that the two recordings should somehow be combined. And although Vampier stated there are no hidden fields in MG1, did anyone actually try to load these recordings as MG1 savegames? It might be a hidden feature in the savegame loader that's been there all along, e.g. something as simple as just displaying a short message on the screen.
Guillian already confirmed the string "BY YUJI HORII". That means an MG1 savefile is out of the question. I'd even assume the tape format is not for MSX at all. Try looking into tape savefiles of games by YUJI HORII, on the NEC PC-xxxx homecomputers.
The tapes are not savegames and can't be combined. The first one contains binary data (Z80 dummy code?) and the second one is some kind of BASIC program (token + ASCII parameter/value).
Some years ago I also decoded all rooms from Metal Gear and I did not find any unknown location.
I have checked again the decoded data from the tape. I found this katakana string "サスペンス アドベンチャー 1" (Suspense adventure 1).
So it makes sense the data is related to "Portopia Renzoku Satsujin Jiken" (The Portopia Serial Murder Case) game.
That game is in BASIC and in CLOAD format, so perhaps the tape is a fragment of that game.
Does anyone has a dump of the MSX version?
http://www.generation-msx.nl/software/enix/port-pier-serial-... this game, right?
@Manuel: Yes, that game. http://msx.jpn.org/tagoo/s_check.cgi?LINE=642
I have checked the PC-88 version and it has a quite similar comment at the beginning of the BASIC program:
サスペンス アドベンチャー ACT1
Author YUJI HORII
Certainly isn't the structure found on MSX "official" tape formats although it does sound like the FSK found on MSX tapes. You can adapt the research I did on CAS files for real tape images. Just remove the header bytes and alignment padding. Also if anyone find an error on my research please be so kind to point it out.
https://publicfunc.wordpress.com/2015/09/15/the-cas-file-for...
If someone needs any help with the less skilled kind of tasks, let me know.
Also, for those of you who have Ground Zeroes, maybe replace the file found in MGS5 with the file found in GZ?
Maybe it will trigger something when the cutscenes play with the new file in it.
Would there be any change by selecting a specific time on the computer?
Well with what Siebe Schuemie said, considering that the planning of this can in no way span past Ground Zeroes, or can it? ( It is Kojima after all.. :3 )
Now I'm the furthest from being an expert about any of this but it seems like Kojima has a bunch of tricks up his sleeve, like the hidden FOB Nuke Dismantle Scene from The Phantom Pain that is speculated to be unlocked near Peace Day.
Just to warn: I had a hunch that the tape was from a PC-6001 (that is Z80-based and uses a variant of Microsoft BASIC), so I asked an retro machine expert on Twitter, and it worked:
https://twitter.com/tiny_yarou/status/645002640288428032
It seems to be a loader for Portopia!
Mystery solved! I’m guessing they just asked around at the office who still had a data cassette laying about, so it’s fairly random.