There is no Service Bulletin that approves the modification of a KX-155 to add a VOR/LOC converter. The main PC boards in the KX-155 and KX-165 are different thus allowing for one to have converter capabilities (i.e. KX-165) and one to not have this capability (KX-155). Any KX-155 that has a VOR/LOC converter board is a Suspected Unapproved Part (SUP) and should not be installed into a certified aircraft.