What does 'full gauge' mean on a J Stevens Tool Co double barrel hammer boxlock shotgun?

Are you leaving out some of the markings? If the Stevens name has the Tool Co part, it should say J. Stevens Arms and Tool Co. The "full" indicates that it has a tight choke to hold the shot in a closer pattern over a longer distance. The "gauge" should be preceded by a number (probably 12, 16, or 20) and indicates the correct size ammunition for the gun.