Looked for my thread pitch gauge to no avail.  Sorry.  You might try spare screws you have lying around. If you find one that threads in go to the store and get one that matches.  My recollection was that it is a fine thread screw??? I'm going to say about an 1-1/4" long.  But be careful because they can go though the headliner if too long. Straighten a paper clip and gently lower it in the hole. When it stops you know you will need  a screw a bit shorter than that. The head was seat tapered, crowned on top and Phillips in type - stainless.

I've found that when dealing with things like the roof rack if you put the screws in a medicine bottle and either tape them securely with duct tape to the rack, or put them in designated special place for such things they tend not to get lost.  But even putting them in a box with other parts and they soon find crevices to hide in.

Try a door mirror screw and see if it works.  I want to say the screws are the same as the threaded inserts look similar. However, I have never tried to swap them. 

Thanks Blaine....I found mine in the secret hiding spot I put them in....lol