My memory is a bit foggy on this, but I think it was at the LA AES show in 1988, that one of the handouts at registration was a little card with a spinning arrow, that pointed to either pin 2 or 3 as "hot". In other words, there was no AES standard yet. I think by the next time the show was in LA (1990), pin 2 was the accepted standard.
The first piece of gear that I encountered in my pro audio career that made a difference, was a Shure M67 mixer. It had XLR ins and outs (at mic level), but you could switch them to higher level and unbalanced, and in that configuration, the XLRs were pin 3 hot. So for that reason, I standardized my own rig on pin 3, wherever I could. Of course that gradually changed over the years.
GTD