SL Postmark Examples
An SL postmark code indicates a Straight Line Marking. In every case the marking must be used to deface the postage stamp. Only those markings reliably reported to have been used regularly to deface the stamp are considered postmarks. The description is followed by its dimensions in millimeters, to the nearest whole millimeter, width by height. On multiple-line markings, the dimensions are those of the line with the ship's name only (typically, but not always, the top line).
"SLK" is a Straight Line Killer, struck alone across the stamp.
"SLP" is a Straight Line Postmark, with the date added by a separate stamp.
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