In preparation for a showdown with the Security Police who control the city of Udo, Chirico (plus Gouto, Vanilla, and Cocona, who joined later to warn of the danger) sneaked into a Scopedog stored in the Battling Stadium warehouse and used some available parts to enhance its firepower.

It's a reproduction of the "Scopedog Tactical Custom" firepower support machine used by the Red Shoulders, and is so heavily armed that an average AT pilot would be overwhelmed.

Vanilla Bertler painted the left shoulder of this machine red in imitation of the Red Shoulders. However, the actual Red Shoulders-spec machine is darker in color and painted in the opposite direction, but this was enough to intimidate the Security Police.

It fought alone against the countless Security Police machines, but was stranded due to accumulated damage. As usual, Chirico abandoned it, and the rest of the battle was carried out using a spare standard-colored Scopedog prepared by Gouto and the others. Also in the story, an unmanned machine (which Chirico calls the Rabbit) with a similar red painted left shoulder but no reinforced armament appears, prepared as a decoy before Chirico goes out on a mission with this machine.

Although this machine has specifications based on it, it was not officially adopted, and the name Red Shoulder Custom is merely a nickname.