Mine don't screw or bolt to anything.

Is this what you have?

All you have to do is take the hinge pin out, put the mirror arm in, and put a new longer hinge pin back in. The threaded hole could be used to secure it to the hinge, but I found that unnecessary. I had to grind the little tab slight as it was too thick but otherwise it worked great.

1946 GMC Pickup - S-10 Frame, 455 Buick, TH400, original patina.

My 46 GMC on Photobucket