Shortly after the german Victory at Tannenberg (late 1914) over the Russians, the Small Arms of roughly TWO Russian Army Corps were re-issued to Units on the Western front, in the region of the Belgian Border with France; the Imperial Navy had all their Gew98s withdrawn, and they too were issued with Mosins, as werre their “Matrosen Divisionen” ( naval Infantry) which also served in the Ypres area.( from Ypres to the sea.).
“Diggers” ( a Webgroup of Military archeologists in Belgium) have in the last few years, un-earthed a variety of Russian made equipment from the Ypres sector, in both purposeful digs, and occasional researches due to Roadworks, Building construction etc, which has unearthed parts of the original trench lines (of both sides). The Nature of the Boggy soil has presereved quite well any brass items, and the Steel rifles etc are also reasonably well preserved. Depending on the Moisture content, wood has also been well preserved ( Framing, Ammo Boxes, etc.).
Later in the war, (1917) there also will appear German-made 7,62x54R ammo, as DWM made sdome specifically for the German Army. But for the majority of the war, the germans relied on the very large quantities of 7,62 captured from defeated Russian Forces on the Eastern front.
There are references to the use of “Russian” and “Belgian” machineguns ( obviously Maxims) by the Bavarian Troops in the area (whose diaries and field reports survived past WW II, where-as the Prussian Archives were mostly destroyed in the final battles for Berlin).
Any (German) Trench system towards the North Sea end of the Front will have evidence of Russian, Belgian and German ammunition use, and late in the war ( 1917-18,) also .303 mixed in with the German spent cases, as the Germans also prized the captured Lewis Guns as well.