The winter game is presumably set in the 1910s-1920s just after the first world war, actually I think quite a few of the games took place around this time period. The most notable evidence for this is of course in our soilders, Martha's uniform matches the womens royal airforce, during ww1 90,000 Gurkhas like Niab served in the Indian Army (Source), and Leo's factory is shown to distribute tanks, something that wasn't used in warfare until ww1 (Source), even Leo's mask is modeled after a tank mask. This is also the time period in which the IRL Servais Le Roy first preformed astrah levitation (Source), though I'll admit basing this theory on the real people characters were based on is a little shaky considering IRL William Webb Ellis died in 1872 only 7 years after IRL Le Roy was born.
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