Maybe call Freightliner Custom Chassis Help line and ask. Have the last 6 digits of the VIN handy. 1-800-FTL-HELP You may be able to order it direct from FCCC Parts dept - I've obtained a couple items that way. I get the part number from the HELP staff and then ask them to transfer my call to the factory Parts people to actually buy/ship it.
I think the parts guy is splitting hairs. The turn signal control is not actually made by Freightliner, but I'm pretty sure it comes as part of the steering column assembly when GBM bought the chassis. There is probably a Freightliner part number that identifies the source they get it from, but the part number on the control itself is likely to be one from the supplier.