Good question about the biplane. I'm guessing that the song was originally published and copyrighted in 1898, but this particular edition of the sheet music was printed later.

I've seen it happen with other songs: sometime after the original publication date, a song has a resurgence of popularity, maybe because it was featured in a new play or film, or a new singer makes it popular again. So the sheet music is reprinted. The cover may be redesigned, maybe to show a picture of a new singer, but inside the cover, the music, lyrics, typography, etc. remain exactly the same. I suppose the publisher saved the original plates.

Furthermore, I'm guessing that a lot of old war songs were revived during WW1, and that's why this edition features a biplane.