Thanks for the answer!
Indeed, the first picture is the hinge that you'd mount to the seat pan.
I'm sorry, I think I haven't been clear enough haha. My suggested solution would be to mount this hinge (first picture) to the seat pan as it would be in any CB500. Then, I'd use the middle hinge (of the second picture) as a replacement for the two tubes on the frame that have been sawed off. So, it would be welded on the frame in the exact location that the tubes would be. Then, the hinge on the seat pan would be connected to the welded hinge on the frame via a pin.
I was just wondering, if this middle hinge (second picture) fits in between the sides of the original hinge on the seat pan (picture 1) and if they have the same pin size.