Yes, they used to have poster "blanks" like that. Senior mentioned many moons ago about things like this in Enniskillen station for excursions variously to Dublin, Derry or Bundoran - I think the destination itself could also be written in. They also had blank luggage labels! If they ran out of a type of label for a particular destination, they would write a blank one until a new batch was delivered.
They used "copying ink" pencils. Anyone here remember these?
They wrote and looked like ordinary pencils, but once you'd written, if you put a damp cloth over the pencil writing it turned to an indelible purply-blue ink. This is the type of thing I would have expected any of these posters to be written with, as if they were pasted outside the pencil would take on the appearance of felt pen once it got damp. Overnight dew would even do the job.