Question I have about the uniform of the Russian infantry. The turn-down collar is new to me. Rather typical were the Russian blouses (Gymnastjorka).
This is not a Gymnastjorka, this is a British style jacket. Such tunics became fashionable among officers of the Russian army around 1915. They were not officially regulated, but, nevertheless, they were very widely worn.
But the Mauser pistol was usually not attached to the belt - it was too heavy for that.