First off, I kind of think I might get told I should've put this post in the similarities thread, but I'm not since I am not just pointing out the similarities, I'm also posing a question about it I'd like the answer to.

So, in 1981, The Michael Schenker Group released their second album, called MSG. On it there is the song On and On. Make note of the intro keyboard melody:
Michael Schenker Group - On and On

In 1995, Bill Tsamis uses a very similar melody for the Lordian Guard song Revelation XIX, found at about 1:12:
Lordian Guard - Revelation XIX (set to start at the correct section)

I'm pretty sure several years ago when Bill Tsamis was more active on facebook, I saw him acknowledge he got this from the MSG song.

But here's something interesting... In 1994 there was a game called Super Metroid released for the Super Nintendo. The game at the time seemed to be at least modestly popular (and has since seemed to have achieved status as a masterpiece of its era). In the game's soundtrack, there is a bit that I think the Lordian Guard bit above sounds even MORE like than it does the MSG bit!

Super Metroid OST - Main Theme (set to start at the correct time)

The Super Metroid game was released in 1994, and the Lordian Guard debut (which is where Revelation XIX is found) was released in the very next year, 1995. I don't know if it's possible that Bill Tsamis was influenced by the game, but it seems plausible to me. But considering he's an 80s metal guitarist, it seems for more plausible he got it from The Michael Schenker Group. But the Lordian Guard melody sounds more like the Super Metroid melody to my ears! And did the composers for the Super Metroid soundtrack, Kenji Yamamoto and Minako Hamano, also get their melody from The Michael Schenker Group? That seems unlikely to me...

So here's the premise of this thread.... is there an earlier source this melody could have come from? Like some classical music piece? Because that would explain this to me.

It's "The Ecstasy of Gold" (1966), a quintessential cowboy theme, at least it sounds like a condensed version of it but the gist is there. https://www.youtube.com/watch?v=nOr0na6mKJQ

I'm stumped. You've obviously done some research. Maybe similarities in the pieces but I don't find them to share the same melody. As far as the origin, I would venture into classical choir. Thats how the Metroid sounds like a digital classical choir piece.

On a side note, I was watching The Good, The Bad, and the Ugly last night.

Yeah, Ecstasy of Gold is probably the earlier prominent example. It's a relatively simple riff based off the same minor scale intervals the 'power chord' is based on, so it's not terribly surprising to see it recur in metal.

That does vaguely sound like the melody (especially the MSG version), but it's so bizarre, the Lordian Guard version sounds the most like the Super Metroid version, and the least like this one.

Holy shit, the Super Metroid soundtrack sounds like dungeon synth.

Okay, as you were.

Nah, not really. It sounds like a standard SNES soundtrack.