If you have a spare half hour, check out this video history of IATSE. It was made by members of IATSE themselves. I’m not entirely sure when it was actually produced, but it had to have been somewhere around the late 40s.
The video features then-President of IATSE, Richard Walsh, as he is visited by the ghost of John Williams, the first President of the union in 1893. The history of IATSE is more than just about the union though; it is also a history of changing theatrical technology from the days of gaslight to our modern world.