Looking at the pictures I have of originals - have to toss out ones that obvious been coated with dealer or aftermarket undercoating
Here is what I found - best I can tell it looks like very little to none
68-Unknown 500 conv
68-202
with a little rattle can thrown in on the frame and elsewhere
68-2669 Can't explain the amount and location. Rest of the car appears to not have any added coating. Passenger side is like all the other cars as appears the rear part of this same wheel well
68-03S173663
68-2985
68-3416
68-3622
68-216161KR
Hope this helps