Re: Canon de 155 L Modèle 1877 and Canon de 120mm Mle1878
Would also add to these excellent suggestions the canon de 105mm Schneider Mle 1913 which served not only in WW1 but was also the principal gun of the French Army in 1940. It was exported widely between the wars to Poland, Yugoslavia and Czechoslovakia and Belgium.
For the Austro-Hungarian Army the 149mm Skoda Model 14 Howitzer would be a great addition.
It was also used by Turkey.