The lines are shadows caused by dampening wires. The 15" have one, 17" & 19" have two, and some 21" have three (I think). The wires are there for stablization of the vertical wires that make up an aperture grille tube. Somewhere on the Sony site is a FAQ addressing this.
And yes, I have a couple of 19" FD Sony's and you get used to them very quickly.