Two weeks ago, I posted a series of battleship margin pieces with mysterious numbers added. While I've learned nothing definitive, there has been some back-and-forth speculation worth noting. Dave Thompson found that the Durland catalog references additional numbers that were added to the plates for the Pan American issue. These were almost certainly added to reduce confusion and error during the bicolor printing process, so they may or may not be related to the added numbers on the battleship plates.
Bob Mustacich offered other possibilities. I'll turn it over to him:
Of the various speculations regarding these extra printed features in the margins, I do favor those built around 2 ideas: (a) these stamps were produced in great haste [while the BEP had started some work in anticipation of the War Revenue Law of 1898, they still only had 17 days to produce and distribute stamps. This was responsible for the familiar shortages and overprinting other other stamps.] and (b) the acceptable production quality may have been lowered for expediency. This situation happened in the production of revenues called for during the Civil War, and errors and quality problems abound. So one speculation is that plates with markings on the edges or abandoned experiments with plate numbering/labeling may have deemed "good enough" under the circumstances. And the same for paper -- paper that had some inconsequential printing in the margin perhaps passed unnoticed in the rush, or maybe this was another short-lived experiment in labeling/marking/number that was abandoned for expediency. The 1/2c documentary pair is identified as plate # 7971, the first plate listed in Durland, so this likely was an early production plate. The items are also rouletted which places them earlier in the production of battleships.
David Thompson sent to me some material from the Durland catalog regarding markings on the plates for the Pan Am 2-color engravings to make sure that the plate combinations and orientations were correct for this more complicated production. I don't know why any of this should end up on the single color engraving plates for the battleships unless they decided to ignore plates that they had pre-marked on the edges for 2-color printings and decided to use these instead for the battleship plates. If they were otherwise good plates, why not? They may not have cared about some inconsequential extra markings on the plate margins.