Gas Cap Help

I have a 1971 dart Swinger, built in canada in April.
The gas cap thats on it, and the replacement I got dont fit correctly.
They seem lose, and can be turned 360 with a little pressure.

Also is it ture that the rear spoilers on the 1971 is differant then other years?

Thanks for the help, I have only had the car for a few weeks, so this is new to Me, and I need all the help I can get, I keep paying for the wrong parts!
Being ignorant is getting expensive!

Bear in MO