Originally Posted by fietsbob

Burn Paint at a Frame builder and take charge of the situation,
with modifications to make it what You Want.

a machined steel block, bore out to 1" , drill and tap 4 holes in the corners,
then split the block thru the bored inch hole to halve it, with an 1/8 inch removed.

re drill the 4 threaded holes on the top half, to take out the threads.

Bore a locating ring in the bottom to make a better brazed Joint than a flat Butt fillet.

and Braze it onto the cut off tube, after prior clamp with 1 bolt is removed.

Or at least that would seem a logical ordering of the steps involved.

then get the whole thing re powder coated.

have to replace the hinge pin..

another method might be to add what is to be 2 hinges front and back to the
existing Clamp. then after welding the sections of the hinge tube in an alternating
pattern cut thru the inside-out just enough
to have the 2 sides open up.
around that Hinge you created, still use the original 1 top Bolt.