I'll have one more go, then I give up.
If it's a
name that links Heflin to a car then I can't find anything convincing in the titles of his films, so maybe it's the name of a character he played. Going through the films where he had top billing, nothing strikes me as a "very well known" name, apart from in
Grand Central Murder (1942) where he plays a character called Rocky Custer (OK, I know General Custer wasn't called Rocky, but it's the surname I'm concentrating on).
And here's a Custer Park Car from 1924.
That's my best shot, I'm afraid.....
