Well Guerra
Since I don't want to expand on this theme, let me just confirm that the use of SB-2 was a mistake made by Western journalists who at the time did not understand what was ment by Tupolev and the designation in question. As far as I know the SB is the only type where the engine version is part of the full designation. That is most probably why so many fell into the trap. By using the type designation SB you don't have to be concerned about which version you are looking at and as such you will never be wrong....
Also if you buy a modern fact book about Russian aircraft during this period you will also get a pretty good answer to your other problems regarding the Polikarpov fighters. Not sure about the Internet/Google here, but you can try that as well.
Cheers
Stig