Though my field is Br. FPOs DC4, DC5, DM3 and DM4 datestamps used by Indian Army units in WW1, but I can tell that the Br. FPO 10 was located in Béthune area in France from January 22,1915 to May 9,1915, serving 10 Brigade of 4 Division.
The type CM3 triangular censor mark with No.512 was used from April 1915 to January 3,1916 by 1 Royal Irish Fusiliers Regiment under 10 Bde. 4 Div., which arrived in France on August 23,1914 in Boulogne-sur-Mer and in April 1915 it was deployed to fight in Battle of Ieper II.
On August 3,1917, the Bn was transferred to 36 (Ulster) Division and on 24 August to 107 Brigade.