Hi Libbygirl,
Just looked at the listing for the marriage on Ancestry. It is definately registered for 1915. As the acronym (1913 Onw) is next to the district I would assume that it has to do with some council rule that came into force in 1913. I also looked on the Freebmd site & found the same marriage listing in 1915 but the acronym wasn't listed. So it's anybodys guess as to why it was put there. The actual certificate might shed some light on the mystery.
Good luck
peta