At the Hoover Political Institute at Stanford University,
Hoover Political Poster Database their library has the following reference:
HOOVER POLITICAL POSTER DATABASE
Poster Details
Poster ID: UK 718
Issuing Agency: Great Britain. Army. Publicity Department
Country: Great Britain
Artist: Sampson,
Text Transcription: Skilled mechanics urgently needed, Imperial Royal Flying Corps. ...
Date of Publication: 1914-1918?
Publisher: Slone, Ltd.
Color or B&W: Color
Width: 25 inches
Length: 35 inches
Description: Drawing of an air battle.
And, at the University of Montreal, in their library they have a collection of war posters that includes the following reference. If you go to their website and search for this poster there is an image of it. It appears in that image that their poster may be cut down on the right side, but you can still see a bit of the 's' What is missing is whether there was a period ' . ' after the 's' that you see in some other posters of the era.
Titre: Skilled mechanics urgently needed : Imperial Royal Flying Corps
Titre traduit: Mécaniciens de talent demandés d'urgence: Imperial Royal Flying Corps
Pays d'origine: Canada
Période: Première Guerre Mondiale
Thème(s): Aviation et Guerre aérienne Recrutement
Identificateurs: Grande-Bretagne. Imperial Royal Flying Corps
Artiste(s): Sampson
Date: 1915; 1916; 1917; 1918
Notes: L'Imperial Royal Flying Corps permettait aux Canadiens de se joindre au Royal Flying Corps britannique. Texte coupé à droite de l'affiche
My suggestion is to contact both libraries and talk to one of the reference librarians to see if they would be so kind as to look at their example of this poster in their library collection and tell you what they see regarding what is on the right hand side of the poster. Clearly there is an 's', but is there a period after it. Also, if your poster is badly damaged they may be able to help you fill in other missing parts.