I'm not sure, but is the ghost piece in Tetris generally attributed to TGM1? Konami's Hexion had a ghost piece way back in 1992. It also had Doubles, and 2P simultaneous single player. It definitely feels influenced by Sega Tetris and possibly influential to TGM. Konami actually licensed the game from David Markley, who created it in 1990 (then called Hextris). I'm curious if it had the ghost piece even then.