Bicycle Bolts (USA) Seatpost Binder Security Bolt

uses a special allen key which is included

length: 17mm,19mm,22mm

It fits many vintage style frames with brazed seat lugs. 19mm is maybe the most common size. You will need to measure your seat lug and buy a bolt that is just a little shorter than what you need so it can compress that clamp. A 19mm binder has an effective clamping reach of about 15 – 25mm. Note that this type binder has a key on one side so that it doesn't rotate while fastening. Make sure your seat lug has a key slot. The “key” and other dimensions are detailed in the technical drawing among this listings images.