Checked my books - Nothing!

I can only attribute this problem to the excellent Chrysler Quality Control in the 70's and the stamping(s) were out of spec. It WAS the end of their production life after all.

Sorry that all I was able to give you was info but no solution.
Like I said, I was able to get mine to pop out with a little pressure. I saw no clips, retainers, or anything that would hold it back.
Try using the holes in the bottom of the bumper where the lamp sockets went through and give it some force from behind.