A neat chart from the Wiki article, of which I was not aware previously. I'd be interested in a similar chart of the pre-1970 board of directors, if anyone were to do the research to get the data. It would be difficult to find, I imagine, but with the WT being a legal entity it is probably available through secular sources.
In practice, though, I think it likely that the earlier board of directors did not really function as the spiritual leaders, voting on doctrine, etc. At least that's the impression Ray Franz gave in his books.