SIG SG 543 Assualt Rifle

A SIG SG 543 is used by one of Wall Streets' assassins to shoot up Frank Martin's house. His SIG SG 543 has a 40mm launcher attached. The launcher for the SIG SG 550 series is the SIG GL 5140 grenade launcher however this is not one of those launchers. Maybe an older launcher meant for the SG 540?