A chrome split ring clamp about 4 inches wide fits around each fork tube.
There are 2 protruding mount points welded to the outside of each ring clamp.
The chrome mounting plates in the pic above bolt to the ring clamps.
The official Triumph 'Fixing' kit includes those fork clamps and I believe also the chrome plates in your picture are part of it, too.
So you are missing parts and nobody but Triumph makes the ring clamps.