The rear bracket looks production, not custom. I suspect it's from La Pera. The hinge pice bolted to the seat looks like the standard $10 DragSpecialties budget seat hinge. The small piece bolted to the frame is larger than the stock bit from the hinge, It's usually a smaller tab curved to weld to a top tube. I've seen some with larger tabs like this but it's on higher end brackets. Either way, easy enough to find or fab, not too much to it. It looks like a budet piece of just steel but I coudl be wrong. The nicer pieces have bronze bushings inside that last longer before they wear and get loose, and the bushings are easier to replace than the steel tube they sit in.
Thanks so much for the pics, it answers all my questions. Now I need to get some $$$ together and stop making foam dust mess down in the garage, This is a nicer seat than I can make and it's not very expensive.
Thanks again,
Scott