Modern US Line Pair Coil?

I don't recall seeing any line pairs on recent coils. This 10 cent clock appears to have a line pair but without the plate number on either stamp.

Better late than never. Line pairs can be caused by different printing methods. A 'joint line' is a printed line caused by two adjoining curved printing plates. They exist on stamps printed by the intaglio method on rotary presses that use multiple line engraved plates.
These lines were evident on stamps printed on the old Cottrell and Stickney presses and ended when the last Cottrells were retired in the mid 80's.
On the pair above the line is called a seam line by coil collectore. This line is the byproduct of offset printing. Ink builds up on the printing blanket from the edge of the offset plates. On some stamps this is very clear, on other issues it is very difficult to see.
On thye engraved plates the plate number was on the stamp just to the left of the line; the 'seam line' is usually further away.

Thanks very much Peter. I had originally posted this in 2010 but received no replies, so with so many new members, I resurrected it to see if I might get an answer this time. I had never heard of a seam line and your explanation is very clear. Wonderful information.

I never heard of a seam line either - thanks! And since it is not intentional or predictable, I assume it will never appear in Scott.

Every EFO listed in Scott was both unintentional and unpredictable.

Yes doug, you're so right. Scott is not really a 'specialized'! But if you are interested in these the Plate Number Coil Collectors Club (PNC3)
does list them in their price list. For more info go to www.pnc3.org.