Watermark Check - The 30¢ Franklin perf 10 was printed on single-line watermarked paper. It is also generally accepted that it was printed on unwatermarked paper. According to Rarity Reveled: The Benjamin K. Miller Collection: "BEP records reveal that four 30¢ plates were printed from March 13 to 17, 1917, perfectly timed to be finished on the last gauge 10 and the first gauge 11 perforators." From these plates, 2 panes and a block from a third pane are known. This stamp is listed as Scott 476A.
There is little point in worrying whether a perf 10 thirty-cent single has a watermark. It has been shown that the layout of the single-line watermarks sometimes allowed for an individual stamp to be missed, while its neighbors were watermarked. Unless provenance can directly prove the stamp came from one of the known unwatermarked panes, singles, even with absolutely no sign of watermark, can not be positively identified as having been printed on an unwatermarked sheet and can not be certified. In particular, a used single has no chance of being certified since its provenance will always remain uncertain.