The lines at the lower left of 6R5.1 have bothered me. They do not look like random scratches or plate damage. Further digging has confirmed the existence of several copies and a plate position (A-10), but no explanation.
Given all the 5 on 6 information that has been determined over the past few years, I tend to look in that direction when I see lines in the upper portion of a Six Cent. (See arrows on 6R5.1full.)
On the far left you will see some red marks near the top of the 5 Cent template. When these are lowered and moved to the LEFT, you will see that they align very nicely with the marks found on 6R5.1. If you look at the lower left detail of 6R5.1, you may agree that the width of the band is the same as the band of colour above the ADA area and that there seems to be part of the space between letters printed.
The shift to the left could also explain the horizontal line in the UR of 6R5.1 as part of one of the horizontal corners of the 5 Cent. This shift would also be consistent with the other 5 on 6 varieties.