If we refer to the copy of f57v that you refer to which is available online at Beinecke and Zoom on the image we can see 4 roughly drawn lines across the borders of the concentric circles, which may be what you are referring to. It strikes me here that if the circles themselves can be drawn so precisely then why aren't the lines also drawn with a straight edge?
Also, on the penultimate circle that you refer to there actually appear to be 2 parallel strokes marked through this circle. Lines drawn roughly from the center of the medallion where "the character immediately clockwise from these lines on the penultimate ring of letters [the ring of 4 x 16 single characters] is always "o" do not appear (unless I misunderstand your directions and am on the wrong folio; note that Beinecke labels this folio f54v).
BTW, if you include the "o" in your count then the penultimate ring of single characters is 4 x 17.