What do you call the right delimiter on the treasury SN for a series of 1902 Date Back or Plain Back? They look like right-looking eyes to me or I've also heard chicken foot. Here's my latest collage on the variability of those eyes (or feet), basically from about 1908 through to 1925. Oh, a useful piece of info is that series of 1902 red seals used a different delimiter and the highest treasury SN on a 4x$10 plate was Y608545... my Warren note shown last in the collage has Y576088.
Most of these notes can be seen in my signature set where I track treasury SN by the back plate number. Look for the notes from sheets of 4x$10s at the end. The Warren, PA notes are fun as the delimiter is very different on each (the V-block awaits the next PMG submission).
I'm not sure, but I don't think paging machines needed the 'chicken foot/eye looking R or L' delimiter until after rotary number machines were introduced in 1903. Someone better at type currency can correct me, but I know that delimiter exists on 1905 $20, and 1907 $10, and 1922 series Gold Certificates, but not on anything earlier?? Before 1903 when numbers were paged?? So when was the first use of a paged chicken foot? I'm thinking it was on a NBN Replacement. Maybe this one: https://currency.ha.com/itm/national-bank-notes/cincinnati-oh-5-1902-date-back-fr-592-the-fifth-third-national-bank-ch-m-2798-pmg-choice-very/a/3581-20433.s?ic4=ListView-Thumbnail-071515#