I remember visions or dreams of Finn with a prosthetic arm even earlier, including Finn’s own daydreams. I guess he thought it’d be badass in an Edward Elric way. Shivers! Danny, have you watched Fullmetal Alchemist Brotherhood? I think you’d love it to a dangerous extent.
I’ve seen a couple of random episodes (I don’t even remember if it was brotherhood or not) It’s to be watched eventually