Look what I just got in the post

Yes indeed the butterfly bridge is different from my other Gazelle, but I've seen ones like these before on Champion Mondials. They threw all sorts of leftover metal at these frames obviously

Dodgy seat clamp bolt is looking a bit crude indeed, but it will be hard to get the metal insert with threading out from the clamp. Maybe I'll just try to find one that's a bit smaller

Still, not bad at all for the price and, Gazelle thread >>>>

The brake bridge is original. This is the older type (2 holes). The other butterfly brake bridge (6 holes) is on newer frames. No idea when it was changed. This orange frame is older than 1976 for sure.

Brake bridges of city bikes and semi race frames were different again, to accomodate the assembly of carriers, locks en fenders.

For the seat lug: drill it ø 8 mm and use a Sugino bolt.