Ok I just got back from my C3 junkyard. Good news Dave, he has what you want. Steve does not know what you call it but when he read what you wrote he knew exactly what it was.
What is it? Steve said he sure that for all 72-74 non-AC cars use the same part. They all had smaller radiators than AC cars and this item was used to make up the difference in the size so that air would flow through the radiator.
Just as you said.
If you are interested the cost would be $40 plus shipping and he will take a picture of it so you can confirm it is what you want before you by it. He just does not want to dig it out unless you are still interested in it for the price he wants.
If you are interested PM me and I will give you his contact info. (Cell phone and email)