It's probably modeled on the similar twin mirror periscopic sight used in the Blenhiem for the lower rear facing turret to protect the tail. The gunner received a correct image of the approaching fighter. His FOV was very restricted. The RAF units which used the B25 found that formation flying made the ventral bendix turret effective in spite of the aiming problems.
The gunner aim view would be something like our F6 bombardier view with either a black cross hair or a black 35Mil and dot ring for the aim point. The periscopic tube was not illuminated.