That's helpful, Scott! If the space is 20mm between the inside of the bolt heads when fully closed, then my guess on the 19mm bolt was right. Thanks!

I'll post pics this evening of this aberration including the seat lug. You may want to wear welding goggles as this build may be NSFF (not safe for Francophiles).

And to clarify:

Top: Common "keyed" binder bolt
Bottom: Campy "knurled" binder bolt