Russia semi-postal, Scott #B15. Don't trim the edges as it was issued imperf. A major problem with this issue is......counterfeits are all over the place. I don't know if the markings at the lower right will help determine is the stamp is genuine or not. Maybe someone else familiar with this issue can help further.
You'll also notice another difference in the two illustrated stamps. Here is my (sold as as a gum-free reprint) stamp with the distinction circled.
That chimney is outlined in Nelson's picture, but shaded in Carol's and mine. It has no effect on value.
I am a trimmer of imperforates, although the triangular mark might stay your scissors. After all, you are only tidying up the scissor-work of the Russian counter-clerk in 1921 - he has no better claim to authenticity than me. Mind you, I trim responsibly...
Of the dozens I have seen of the 3 colors - none have had that mark.
Overall the most distinguishing feature of the forgery is that it is better than the original. Not all forgeries are crude.
The details are much clearer.
Forgery on top
Edited: When looking for forgery details, it is important not to be fooled by plate flaws, features of plating positions or inking (over or under)
Whatever the detail - it needs to be consistent - hence the challenge
