Hi Rob,
The pictures of the original engine bays I believe you are referring to, does not show the rubber plugs because they are painted, but because there are no rubber plugs in the inner fender sidewall on the earlier 280s. But if your car has them, they where as you noticed, originally painted. No 230 or 250's ever had those holes/ plugs. Trying to adjust to the judges detailed knowledge or lack thereof will be impossible.
I agree with Gael, be confident and do it the way you know is original and correct way.
Hans