I've only collected one ad on the plastic backed and faced composite of Eicholtz. Eicholtz was referring to "plastic" rather than "fiberglas."
My take is that he was simply experimenting and it didn't pan out. There likely were very, very few made, but it would be cool if one still exists. Eicholtz didn't give up working with fiberglas.
"Fiberglass, (also called fibreglass and glass fibre), is material made from extremely fine fibers of glass. It is used as a reinforcing agent for many polymer products; the resulting composite material, properly known as fiber-reinforced polymer (FRP) or glass-reinforced plastic (GRP), is called "fiberglass" in popular usage."
Fiberglass is basically a plastic. Plastic didn't work well as a backing but glass reinforced plastic or "GRP" did. Plastic did work for facings but fiber-enforced polymer or "FRP" worked better.
Eicholtz's "Conolon" backing was a GRP and W. L. Miller's "Toxhorn" facing was a FRP. By 1952 or so, the quality of "GRP" had evolved so that it worked best for both facing and backing.
The differences between what Miller and Eicholtz were promoting was in the "fiber" and usage. Miller was the "facing" guy and "Eicholtz" was the backing guy.
Miller later did add a line of fiberglas to his products.