66 Belvedere II Fender Tag Decode Help

[Nxcoupe]

Figured out everything except what d6 means? Top row. Any help would be appreciated. Thanks!

 

[R413]

426 rear speaker fader. Should be a switch next to ignition key, or on other side of the column. I’ve seen them both ways on factory cars.

 

[Nxcoupe]

426 rear speaker fader. Should be a switch next to ignition key, or on other side of the column. I’ve seen them both ways on factory cars.

Thank you.