Weird Schwinn Notched Seatpost Binder Bolt
 

This is the original on my '86 Giant-made Schwinn World. The threads were a bit rough, so I cleaned them up in both the nut and the bolt. The threads are standard M8-1.25, but the hex heads measure 1/2 inch. I checked it with a metric 13mm, but it was too wobbly. Definitely 1/2 inch. Length measures closer to 1-9/16 inches than 40mm.

There's a slight bend in the bolt, so I'm wanting to replace it. Ideally, if I could and keep the look original and replace it with NOS, I would. Is that an option?
https://cimg3.ibsrv.net/gimg/bikefor...471d450213.jpg
https://cimg4.ibsrv.net/gimg/bikefor...8c28441993.jpg
https://cimg5.ibsrv.net/gimg/bikefor...47b7cbc1f2.jpg

I personally would just be reusing that one.

https://www.ebay.com/itm/36317350358...caAh4JEALw_wcB

I agree. The bend is very minor and the bolt isn't likely to break. Any 40mm M8x1.25 or M8x1.0 bolt and nut could be substituted but finding one with that locator tab under the head is unlikely and, if appearance is important to you, you matching this one is even more unlikely.

You probably aren't going to keep the look of the original. Unless you are just talking about it being a external hex headed bolt.

When younger, I always tended to overtighten my seat post clamp bolts and they'd eventually break. So don't get too ham handed with it.

You mentioned the notch, but you didn't seem to have a question about that. Though I doubt you find a bolt with the tab to fit the notch.

The tab/notch only prevents the bolt turning as you tighten the nut. Without it, you’ll need a second wrench to hold the bolt head - minor inconvenience, but nothing more (so any suitably-sized bolt will do).. However, I’d reuse the original