At the Russian website www.battlefield.ru, in their gallery of knocked out Russian vehicles, there is a photo captioned T-34-57. (http://www.battlefield.ru/destroyed/ussr/t34_57_01.jpg )
The vehicle does appear to be armed with a long, narrow main gun. Does anyone out there know if this is the case? Were there 57mm armed T-34s in use on the Eastern Front?

Well, nevermind. I have answered my own question. There WAS 57mm main gun T-34s used on the Eastern Front. For those who are interested, check out this website:
http://www.battlefield.ru/t34_57.html

Yes, such a tank was produced in small amount(some two or three hundreds, IMHO). It was used in the battle of Moscow. the tank, equipped with this gun was thought to be used as tank destroer mainly. Really the 57mm long-barelled gun was pretty good for anti-tank purposes, but not sufficiently powerfull as 75mm once while shooting usual barrels. The gun iz slightly converted Zis2 once. The same gun was used on some types of anti tank self-propelled guns, as well.