The original gun set-up had flash suppressors on the muzzles, so the outboard guns muzzles just protruded from the gun ports, due to the narrower chord in this area.
The suppressors were later deleted, leaving the muzzles inside the wings, so cutting off the kit part is correct. I think this is probably another case of a manufacturer going off scale plans, or a particular preserved example. Certainly by the time of the BoB, and even earlier, and from then on, the muzzles were inside the wing, with the fabric patch being flush with the wing leading edge.