Just over 30 years ago, I had two 1970 Dart two doors. The driver was from Windsor & the parts car was from LA.

I do not remember now which car had which, but I was taking parts in better condition off the parts car to use on the driver. One had plastic rear extensions & the other had pot metal.

At 16 years old, I would not have known to check part numbers or noticed if they had ever been replaced before I owned the cars though, I just thought it was odd.

The parts car came with no keys, but the keys from the driver car worked in it, ignition & trunk both which was a cool coincidence.

My '71 Demon (6/71 door sticker, 528 fender tag) has fiberglass extensions. I sanded the paint off in the last week and only found the original GB2 and a similar repaint color the nose received at some point. The trim mount is 3-1/4" deep unless you measure at the point, then it's about 4". Born at Hamtramck, lived it's life in Portland.

April 71 built Demon, glass extensions. 100% original

I parted a 71 and it had fiberglass ones....

I will add to this post since I am researching it myself. I have two pair. One is from a 72 demon 340 car and are made from pot metal and use the shallow trim. The second set is from a 72 swinger and are pot metal and use deep trim. In both cases I am quite certain these pieces are original to their cars.
From what I have read so far there does not seem to be any pattern on what was used different years and models.

I had a '71 Swinger built with SPD of 802, VIN# 100331, built in the Ontario, Canada Plant and it had pot metal extensions. I do not recall which style trim it took, but I assume it was the deep style.