The bolts turned out to be a pair of (rather mismatched) hex 1/4" bolts and nuts.
One was a rather long 1/4" whitworth bolt and nut, and the other a fine thread shorter bolt with nut.
I haven't quite got the shorter one off yet, it needs to soak - the hexs are rather weak, so not easy to get hold of.
The plate on the frame and the clamp plate are both quite thin, so it won't need much of a bolt and nut to retain them.
You may be able to just see the plate on the frame tube turns down front and back, and is profiled to fit onto the curved frame tube.
And then brazed (?) there.
Image tags added - Dave , 23 Apr 2020