What a cool car...thanks for sharing the pictures.Pretty neat to have the original title signed over to you as well.
Going by the fender tag,your Duster just had it's 40th birthday on the 8th of this month so you might want to bring it slice of birthday cake soon.
The L4 interior code is for the up-option split-back bench seat with the folding armrest which comes with two tone upholstery and is black/silver (X9) on your car.
There's some other nice options on it as well,with the tach,dual sport mirrors,tinted windshield,chrome exhaust tips,and light package.Looks like the tips have been replaced somewhere along the way,I wonder if the original owner might still have the factory ones...
According to Chrysler literature at the time,only 340 Dusters with 4-speeds came with dual point distributors that year so the automatic cars would have the single point unit.