Inner Door Panel Trim Screws

[Chuckin]

I thought this would be so easy to figure out, but I have come up empty handed in my research. The upper door trim that the door panel fits into.... What size/type of screws/nuts hold that piece on? Obviously, it doesn't really matter as long as it works, but I'm curious what the factory did. There is just a hole in my doors, with no threaded backing of any sort. There needs to be some sort of nut on the back, but all the places I look, the screws look like short wood screws with a flat head, and dont come with any nuts or clips. Thats NOT going to hold that on there tight enough...

Any help would be appreciated.
Thanks