Hendon Poster

I’ve got a couple of copies of this poster but need some help dating them – the serial code at the bottom said 1914, which I thought was the date (and explains why ‘loops’ and air races are the main attraction), however the ‘Hendon’ in the London Underground style roundel is causing me a bit of confusion.

The reason being that furthest the Northern Line went in 1914 was Golders Green, it was extended to Hendon Central in November 1923 (but as far as I know was only ever called ‘Hendon Central’, not just Hendon as on the poster) but that was only the closest Underground station for a few months when Colindale was opened in August 1924 – so, either the poster was published between November 1923 and August 1924 at a time when they were usually issuing much more colourful posters relating to the RAF Pageant showing RAF aircraft blowing up Bridges or Arabian towers, or it is from 1914 and the likeness to the Underground sign is just artistic license – any thoughts?