Oh that's great about those home made benders. Those things are 100.00 plus tax here. I'm wondering about a home made end former: I can see that the tweaked end of the spring is part of the way everything works together.
It looks like the hole for the push pin in the back of your cover is kind of stretched out. And is it a black cover that's been painted? If so, it's likely aftermarket. The good news is that you can get the same ones and paint them again. I found these ones but I don't know if there are others. They come with the pins. They just seem to stay there once on the hinge. The one end is like a foot in a slipper.
https://www.ebay.ca/itm/1966-72-Chev...5:pf:0&vxp=mtr
I have yet to strip the good seat back and the base to get you a proper dimension and the detail on that seatbelt passthrough. I can tell you that it wasn't east to pass the buckles through when I was putting the new seat in, and it was a real bugger trying to push them out when I was removing the old seat. You really need a lot of patience but they will go through. The factory must have had some kind of tool to do this. Also, probably easier to do it when the seat is out of the car and then just bolt the belt ends at the floor after.
It looks like the replacement spring sets are discontinued. Bummer. Oh well, the springs from my local upholstery shop are 4 bucks each.
https://www.ecklerschevelle.com/chev...1964-1967.html