Is it a clip or nut sert for reattaching the mirror?

Well, your going to wish you asked that question before the car was painted…
The door should have threaded nut-serts.
They can still be added.

The article at https://www.hamtramck-historical.com/mirrors.shtml may help you.

I was in same situation, just used SS screws with washer and nut on back.
They only used nutserts from factory because it was easier.

And they had special oval headed screws.

Yep, available at ACE HW

Use the nutserts. They are easy to install and if you need to take off a mirror for whatever reason you don't have to get inside the door.

Here’s a handy kit that includes a installation “tool”. I used it and it worked well.

https://www.ebay.com/itm/294919506933?

Mc Master carr has a nutsert that has a serrated exterior that keeps them from spinning

Ace hardware has the same installation tool as the one in the ebag link, AND it's only about $3.00 or less !

there is suppose to be a reinforcement bracket put under the door with threaded studs attached held in with a rubber threaded bolt.at least on 71 and up cars,dont know about 70s

My original 1972 barracuda has nutserts from the factory. I bought it new.

No, they are different. The originals fit in the recess better than hard wear store china stuff and the philips slot is a little different.