Translating the above to identifying technical specs per vehicle type (limited to C30 and C60S as this the object of our discussion):
C30:- Front axle with 5” steering ends
- Brakes, front drum: 14” diameter, 2” width lining
- Brakes, rear drum: 15” diameter, 3.5” width lining
- Wheel size: 16” rims
- Differential gear ratio: 7.16 to 1
- Rear spring leaves: 12
C60S:- Front axle with 6” steering ends
- Brakes, front drum: 15” diameter, 3.5” width lining
- Brakes, rear drum: 15” diameter, 3.5” width lining
- Booster actuated brakes
- Wheel size: 20” rims
- Differential gear ratio: 7.16 to 1
- Number of rear spring leaves: 12 + 6 Aux.
So the Chevrolet I am trying to ID is a C60S, by virtue of having:
- 6” steering ends - see the picture with six cap screws
- 12 + 6 Aux. rear spring leaves - see the picture showing the auxiliary springs
- True, it is fitted with 16" rims, but it is possible to fit these on 15" diameter brake drums (C15, C30 have 16" rims on 15" diameter rear drum brakes)
Now, if I check if the subject vehicle has (had) a brake booster, I am sure this is a C60S rather than a C30.
Riddle solved!