They would all be Tula, if I remember correctly (I haven't looked up the exact date) Izhevsk didn't start making Nagant Revolvers until the 1940s. The 1916 and 1913 have the potential of being a single action model, which in itself is pretty cool.

Personally I would go with the 1913 or 1916 if they are all correct as I currently collect WWI firearms. That being said 1919 is a very interesting year as the revolution was going on and production likely wasn't very high. If I was still collecting Sovieticon arms, that would be a jewel to own.