Continuing the theme...

Yes, the 15k is a special case, and quite easy to spot genuine/forgery- for genuine, the vertical stem of the "T" of "ATRAD" is thin, the serif at the top of the Cyrillic "D" of "ATRAD" does nor touch the pointed tip of the letter, and the left hand of the man is not hanging down as much.
But what about the other four values?
Varro Tyler (Focus on Forgeries c2000) suggests a good look at the woman's face....

Genuine

Closeup genuine Woman's face
Short dash on the woman's chin which does not touch the point of the chin.

Forgery

Closeup Forgery Woman's face
In two of the four Forgery types, a spot of color touches the woman's chin at the chin point. (Shown here)
Third Type Forgery- a dot or dash does not touch the chin line, but there are faint guidelines visible between stamps.
Fourth Type Forgery- No dash on the chin.
If I may make a general comment about showing genuine/forgery stamps-
It is helpful if the stamp image is at a high resolution, or perhaps show part of the stamp in high resolution- just saying.
