I seem to remember some horseshoe shaped item, but all my current searches show that for fitting v-brakes to caliper, not the other way round.
Just a way to fit a mudguard ?, on those fattys I note there is a bolt
into the end of the fork steerer..
you can use the V brake mounts as a small front above the wheel, rack mount
and , perhaps, that bolt would be a 3rd mounting for that rack
a plug, perhaps another star nut in the bottom of an open fork steerer.. would do similar.
I fitted a small rack to my otherwise lowrider , fork , though
it was not the fork you show.
a decent shop can fabricate one .
Mine was a fabrication at M Steel/Joe Waugh , in the north, Newcastle..
i was on my way to the Ferry Port , to Norway , Mid Bike Tour,
so they did a quick , but solid job.
once the rack is there it can also mount your mudguard..