R147 Imprint?

I picked up a pair of R147 and found the top of the letter"P" ? below the left stamp. I have seen plate blocks from this sheet and they do not show anything like this. The "P" is very similar to the "P" in "Plate No." found on some revenue sheets but not on this sheet. What am I looking at ? Karl
Plate block is on ebay at 141445682064

Inasmuch as the steel for the plates was scarce they might have been reused, and this might be a holdover that did not get burnished. There are a number of examples on first issues. There would not have been more then one bottom imprint, and there were not enough stamps issued between the second and third issues for there to be more than one plate (although they would have reused the number if they did need more). Only 64,368 second issue and 56,185 of the third issue were produced. With each sheet having 72 positions (12x6), it would have only taken about 1675 impressions to produce them, a very low number for a steel plate.