If it helps with figuring out where the header for those special "AtGames/Firecore only" games comes from, it definitely comes from (at least a prototype of) the homebrew game "Mighty Mighty Missile!" for the Genesis. I was able to confirm it by looking at the game's TCRF page: https://tcrf.net/Proto:Mighty_Mighty_Missile! Really, it's just the fact that "FVRING" is mentioned right there on the game's title screen.