Freemasonry does indeed involve a great deal of symbolic stances and signs, and the 'hidden hand' is one of them. Remember when you were a kid in Karate-Class, and every time you advanced a belt, you had to perform a 'kata'? Show off your moves, do a choreographed little 'dance', bow at the start and bow at the end, ect.? Freemasonry has a somewhat similar, if not significantly more gentlemanly and less physically demanding policy toward advancement. Considering there's over 30 levels of advancement in the society, there's plenty of room for handshakes, poses, 'dances', ritual words and hand signs. That'd be why little coded 'cheat-sheet' booklets are employed, so that members can keep up with all the hoopla.
If a majority of the figures with portraits in that pose are in fact associated with or members of the Masons, then that's not an unfair line to draw at all. If they aren't, and only a small few of the portraits in that pose are of people associated with the Masons, then perhaps it was just a 'trend'. Either way, this is the sort of thing that's going to be rather difficult to prove. Stalin might have been going for a smoke.