Mirrprs - Threaded holes ?

[mark '87 930]

I just got a pair of period correct mirrors for my new (to me) truck and am curious about the mounting piece of it. The bolts I received (3) doen't seem to be metal screws at all, and in fact look like they are to be threard into a nut. Should there be holes with welded nuts in behind the door panel for the original mirrors? I took off the interior door panel so i can peak into the backside of the door and there are holes there that have been brazed over to fill them.. but i don't see any evidence of nuts being welded in place in the past.

If you guys need pictures, I can post some.