FSM for '67 curiosity- missing chapters/volumes

I imagined that the FSMs for dealerships were set-up like encyclopedias. Grabbed the volume you needed and went to work. So, I'm curious as to what info was left out for the FSMs the public can buy. Specifically, my '67 FSM is missing chapters/volumes (not sure which label is correct) #s 4, 10, 12, 13, 15, 18, 20. Can anybody shed some light on this?

Viewing the Chrysler parts books and FSMs together, this is what you will see, at least during the muscle car era, with parts book sections not in FSM in red:

Parts FSM

0 Lubrication and Maintenance
1 Accessories
2 Front Suspension and Steering Linkage
3 Rear Axle
4 Brake - Parking
5 Brakes, Service and Parking
6 Clutch
7 7 Cooling Grille
8 Electrical
9 Engine
10 Engine Oiling
11 Exhaust System and Intake Manifold
12 Fenders and sheetmetal
13 Frame
14 Fuel System
15 Hood
16 Propeller Shaft & U Joints
17 Shock Absorbers and Rear Springs
18 Standard Parts (fasteners, fittings, etc.)
19 Steering
20 Unused
21 Transmission
22 Wheels, Bearings, Tires
23 Body & Frame
24 Heaters & Air Conditioning

Thanks for the information.