Great story and post up more photos!
That’s a real deal 360 4spd Duster!! Later model year build to with a scheduled production date of April 16 1974. Your Dad probably bought it in the summer of 74?
M26 is the code for wheel lip mouldings. Hard to tell if that’s a 25 or 26 under the screw. M25 is sill/rocker mouldings.