If you look at Bob Beers book on Aurora cars you can find out how rare a particular body might be. If a body turns out to be rare a collector might be willing to pay a premium for it.
You might post some pictures of the cars.
cigar box and speedsters
many(most) came without windows.
in some cases the mounting posts are different heights than the t-jet.
most are considered desirable.
being a Willys fanatic, I like the Willys cigar box/speesters that have no rear push bar(bumper) or license plate holder on the trunk.
be careful, there are near perfect copies made by H O Detroit that mimic the originals.
after complaints H O D was included inside the bodies.
early ones do NOT have that.
I collect both.
actually, the H O D bodies are more expensive now.
I think the original post is referring to the rarity of an Aurora push car being on a plastic chassis as opposed to the normally seen metal chassis. I would think a car on a plastic chassis would be somewhat uncommon. I have seen a few, mostly early to mid-sixties cars.