Has anyone seen these before.

In the "Kit Car" days if you were running the 1/4 mile ovals you'd want the 'Tri-Y' for the lower RPM torque band. You did not have time to shift gears so a wide torque band was what was needed.
But the 1 mile+ ovals the higher RPM's let the horsepower of the standard 'Step' headers work. You shifted gears so you were at high RPM most of the time.

I had a set of the stepped tube 340 'Kit Car' flat 4 into 1 collectors like those 351C Pinto shaped ones above for a long time but they did not fit 1965 Dart GT 4 speed so I off loaded them CHEAP. I got a set of "WORTHLESS" 340 A body Iron logs in a trade when everyone was tossing those in the trash.

Those Kit Car headers were so much overkill for my 273 but back in 1975 we did not know. I think the main tube started at 1 7/8 inch. The Tri-Y would have been much better on a 273-318.