The following figure provides an impression of union membership numbers for a selection of OECD countries. The historical data for Australia, Canada, Denmark, Germany, Norway, Sweden and United Kingdom come from Bain and Price (1980), the historical US data is from Freeman (1998), data for Austria, France, Italy and the Netherlands come from Visser (1989). Data as of 1960 is from OECD (2011). For references, please see the paper.
The data is available as historical data only and in the version as used for the above figure. I hope all copyrights are respected by providing the sources.
Extracting stylized facts
Here is our result of fitting time and time squared plus a post 1959 dummy (to take the switch to OECD data sources into account) to this data. It clearly shows the inverted U shape and the fact that the peak differs across countries with respect to when and how high it is, as stressed in the paper. The exact specification is available as a STATA do file and the results are in a log file. The STATA data file is here.
This figure contains one country more (Netherlands) than in the paper. The only reason is that one country less gives a nicer picture for the paper. Here we keep the Netherlands for completeness sake.
A more colourful version of this figure is as follows.