Please provide a scan. If part of the perforation row along the bottom of the plate block is perf 10, then the affected stamps would be classified as Scott 512b "perf 10 on the bottom". The perforation row has a distinctive set of characteristics. A large image of a block of 25 showing the left transition and most of the row exists on the Philatelic Foundation web site for cert 446197:
http://pfsearch.org/pfsearch/images/446/446197.jpgCountry: United States Of America
Cat. No. Issue Denom. Color
498g 1917 1¢ green
BLOCK OF 25 WITH PLATE NO. 13366 AT TOP RIGHT, FOUR ERROR STAMPS WITHIN THE BLOCK, POSITIONS 9 AND 10 ARE PERF 10 AT BOTTOM, POSITIONS 14 AND 15 ARE PERF 10 AT TOP.
AND WE ARE OF THE OPINION THAT
IT IS GENUINE, NEVER HINGED, POSITIONS 10 AND 15 PERF. 10 BETWEEN; POSITIONS 9 AND 14 WITH TRANSITIONAL PERFS BETWEEN, THREE PERFS AT LEFT PERF. 11, BALANCE AT RIGHT PERF. 10. * * * * * * * *
A 511a block of 30 containing the complete row also exists on the PF web site for cert 478705:
http://pfsearch.org/pfsearch/images/478/478705.jpgCountry: United States Of America
Cat. No. Issue Denom. Color
511a 1917 11¢ light green
511 1917 11¢ light green
TOP RIGHT CORNER MARGIN BLOCK OF 30 (10x3) WITH PART ARROW & PLATE NO. F13467 IN TOP MARGIN, PERFORATED 10 BETWEEN POS. 17-18/27-28, TRANSITIONAL PERFORATIONS BETWEEN POSITIONS 19/29.
AND WE ARE OF THE OPINION THAT
IT IS GENUINE, NEVER HINGED. * * * * * * * * *
Clark