Here are pictures of two 1970 340 swingers That I owned in the 80's The third picture is a 340 swinger I sold with a deck lid on it from a Dart Custom. Swingers never had the chrome across the deck lid. And I owned many of them. Yours did, So I would say either your car started life as a custom or it had another decklid installed sometime in life. I had done this to some in the past and added the Chrome on some builds. If it was the only deck lid I had at the time