Hello I am the owner of this gun. Previously, it belonged to my grandfather. He worked at an arms factory in Izhevsk from 1933 to 1948. He was a specialist in the production of artillery systems. (In 1933 he graduated from the Military Technical Academy Dzerzhinsky. By that time it was very serious education)
According to these links, you can see pictures of this gun before and after restoration.
http://alexey-klyuev.livejournal.com/8932.html
http://alexey-klyuev.livejournal.com/7754.html