On my 2023 Road King special I wanted to upgrade my rear turn signals to the Custom Dynamics LED. When I spoke to the tech support they said because the bike is “International specific rear signal assembly that does not allow for the replacement of rear signals. If you wanted to swap these signals out, you would either have to completely replace the rear signal assembly, or swap to a US style rear signal bar.”  

Hi bukkers there's probably not many on this site with a '23 RoadKing but have you enquired with custom dynamics themselves or even a American harley site like HDforums?