So did the Plymouth Valiant name come from a french cartoon character?

For the record, "Signet" is not a made-up word.

signet |ˈsignit|
noun historical
a small seal, esp. one set in a ring, used instead of or with a signature to give authentication to an official document.

The name was probably intended to add some "class" and upper-crust appeal to the top-of-the-line model.

Boy was I wrong, I thought it was a baby Swan!!! (Cygnet) Doh!