Let’s be aware of undercoated cars and non undercoated cars from the factory. The non undercoated cars still had the 4 wheel wells and underneath the front fenders done.

Then we have undercoating done at the dealership. This is where you see it at different places on different cars and can cloud the discussion of what was factory sprayed or not.

There is no factory undercoating on the seat studs and nuts. There is a tape outline on the floor covering the seat holes. The seats were one of the last things to go in the car. After the car was running and driving.
No factory undercoating on any of the front suspension, engine/trans/exhaust/trans crossmember, leaf springs/hangers/shackles or rear axle and brake cables. Not on the car yet.

Fuel tank, fuel lines, brake lines, and the bolt on dual exhaust bracket were on the car when undercoated. Also all body panels and bumpers were on the car. Along with wheel well and rocker mouldings.

I should clarify that my observations come from 67-70 B-bodies from St Louis and Lynch Road plants.