Look here for some background information on the Western Cartridge Company made 11mm Gras ammunition contract with France in WW1: histavia21.net/amaviapag/11_Gras-avia_US.htm
I just found some more information about the 11mm Gras cartridge you are seeking info on in Les Cartouches Du Systeme Gras (Cartridges of the Gras System) by Mention and Ramio, 1988. On p. 141 they have a listing ‘Jacketed Bullet Model 1879/83 Cartridge’, this listing includes 3 different HS for Western and one for Frankford Arsenal. This jacketed bullet loading is what is discussed in the web page I listed above.
On p. 142 is the listing for ‘Lead Bullet Model 1879/83 Cartridge’ and they show the headstamp of the Western cartridge you have and an additional one with the HS- WESTERN 9 - 15. They state these cartridges were loaded ‘for the test firing of Vickers guns in 1918’. Just as Fede said in his post above!
To expand upon what Fede said in his post above on p. 120 of Les Cartouches Du Systeme Gras there is the following information: "To complete the test firings of the converted Russian Vickers guns, a lot of lead bullet cartridges was (were) made up at Frankford Arsenal in October 1918. This bullet had a hallow base, and weighed 24.60 grams…Frankford Arsenal , due to case shortages, used what ever where available, which explains the variety of headstamps encountered, like “WESTERN 1915’ with small primer. Although this model was not entirely satisfactory, production continued up to 1919”.