You said there were nuts under the rear beam bolts. Can you feel up there or use a mirror to look under the saddle and tell if there's a plate glassed in. Maybe the bolts were stripped out there also and someone just used nuts because it was easy. I doubt very much that the thing was held together that way, just nuts.

I would clean out the hole. Use a brush and some compressed air. You should be able to tell if it's stainless or aluminum. Then you can plan the next move.