The seam width is approx 1cm.
Ideally there would be nothing there but all basket windshields have to have some sort of physical structure. In practice the polar diagram is is not greatly affected by such elements. You can measure it, and we have, but the variation in direct sound pick-up is very small - of the order of a dB at worst.
More apparent are the diffraction and reflection effects off all the structural elements - basket, suspension parts, etc - which are not the same for all angles of sound pick-up. That creates slight changes in colouration as you rotate the assembly. This is true of ~all~ such windshields and affects only the higher frequencies, but the "ripples" in the angular response are once again very small. They are normally inaudible in absolute terms - most microphones have greater inherent colouration - but may be discernible with critical listening as a relative change of tone when panning the mic in particular situations.