How do you mark your plate holders? I print small labels (ca 1,5 x 3 cm) on thicker paper, put Scotch Magic tape on the side I write on, and attach them to the holders with double sided tape. It works nicely. With a pencil I write the film type they are loaded with, and after exposure I mark them with an "X". It's easy to write and erase on the matte tape surface.