IIRC, it's more the cross section that would be interesting to see (vs the slots in the surface), bends that attempt to better equalize the path delays for a "coherent" summation at the phase plug exit... where the circles you see are like small concentric horn exits that have the 'slots' as their throat. Kind of.

-sort- of like equalizing exhaust header pipe length with the proper bends. I expect someone will beat me up about that ... lol.